* Implement syscall readlinkat

* Implement syscall readv by read syscalls

Since pk lacks kernel-space dynamic memory management, we implement readv with
normal read syscalls rather than forwarding it to spike

It was originally added as a temporary hack to hide a race condition in
a prototype.  It should've been removed long ago.

Resolves #312

This reverts commit 54de960a.

See #298 for explanation.

There are many issues: 260, 285, 287 has to be solved

This workaround helps to add neccessary zicsr and
zifencei for cssr and fence.i accordingly.

Linux kernel simply return current brk when request brk addr is not
feasible. The pk should probably do the same.

This reverts commit 7ae86fb9.

This will continue to allow accesses to cycle/time via mcycle/mtime
despite https://github.com/riscv-software-src/riscv-isa-sim/pull/1297.
The hope is this will keep most people happy while doing the right thing
with Spike.

Regenerated the configure file using autoconf 2.71 to avoid the Syntax
error problem when running on OSX/AArch64.

Fixes #282

We previously kernel-panicked because that made it more obvious when a
syscall implementation was missing.  These days, it's more common that
the C library will do something sensible in response to returning -ENOSYS.
Favor that approach to avoid frustrating users.

The UART drivers all return -1 if no character is present, and so that's
what we should do if there's no UART at all.

See discussion on https://github.com/riscv-non-isa/riscv-sbi-doc/issues/82

See https://gcc.gnu.org/onlinedocs/gcc/Local-Register-Variables.html#Local-Register-Variables
"Other than when invoking the Extended asm, the contents of the
specified register are not guaranteed."

Also revert 90191ad9.

The memory manager maintains the first free page as the page after the
`_end` synthetic emitted by the linker.  This value is stored in a
translation unit local variable.  This value is only ever written to
from `init_early_alloc` which is static and only ever invoked from
`pk_vm_init`.  Furthermore, the value that `first_free_page` is ever set
to is computed as a rounding of the _address_ of `_end`.  Because the
address of the symbol cannot change during execution of a normal
program, this is effectively a constant, making the computed value a
"constant" which can be re-materialized.  Now, with the knowledge that
the value is effectively a constant that can be re-materialized and the
fact that the value is ever written to at a single position, we can
simply re-materialize the value if it was ever changed in
`free_page_addr`.  This will allow the 8-byte value to be truncated to
1-byte.

Now, we can inline `__early_pgalloc_align`, and because the combination
of `__early_alloc` and `__early_pgalloc_align` is small, we can inline
that again at the two sites locally.  This changes the
`__augment_page_freelist` to re-materialize the constant when needed for
the allocation.

The re-materialization however uses a pc-relative addressing, which now
computes a different value than expected - the address has become a VA
rather than a PA.  This results in the address computed by
`free_page_addr` (which is the result of the `__early_pgalloc_align`) to
be a virtual address after the relocation, which then propagates through
`__early_alloc` to the value in `__augment_page_freelist`, which is then
consumed by `__page_alloc`, which will treat the now VA as a PA and
perform an additional translation to a VA.

Mark the value as `volatile` to indicate that the value must be read at
all points to thwart the size optimization of the compiler resulting in
a mis-compilation resulting in the eventual invalid memory access during
the `memset` that follows the allocation.

Thanks to @nzmichaelh for the help in tracking this down!

Eliminate excess reference count when associating fd with file.

Fixes #258

h/t @jrtc27

This reverts commit fd2ddce5.

The SBI took a different approach (explicit SBI call) to support writing
the counters, rather than using traps.

`SYS_getcwd` is different from `getcwd` in that the return value is < 0
on failure otherwise it is the length of the string.  The proxy kernel
was treating 0 as success and all other values as error.  As a result,
we would never return a valid value for `getcwd`.

The following program now executes properly with the Proxy Kernel:
  ```c
  #include <stdio.h>
  #include <stdlib.h>
  #include <unistd.h>

  #include <linux/limits.h>

  int main(int argc, char **argv) {
    unsigned char buffer[PATH_MAX + 1] = {0};
    if (getcwd(buffer, PATH_MAX))
      printf("cwd: %s\n", buffer);
    return EXIT_SUCCESS;
  }
  ```

Resolves #249

Also use pointer arithmetic on char * instead of void *.

This reverts commit 5450c2f7.

This reverts commit 717702ce.

This reverts commit 17bec41e.

Also use pointer arithmetic on char * instead of void *.

This reverts commit 0d1fdc2e24b7b6247a55d24c13ae85dca7f45695.

clang only supports register variables if they are declared globally.

This reverts commit e8d15a48.

Resolves #216

Use of asm aliased register variables in local scope can only be used
for extended assembly parameters.  This changes the few instances of
this in the floating point emulation to use the GNU extended assembly
syntax to access the `tp` register.  This ensures that we do not rely on
undefined behaviour.  This was uncovered when building the Proxy kernel
with clang and LLVM.