Modify glibc files to cooperate with Linux/RISC-V

* Tweak Makefile parameters and correctly inhibit libc.
* Disable bootstrap on both stages' builds of riscv-linux-gcc.
* Adjust the #define for ABI_64 from 2 to 3, because glibc assumes
  that _RISCV_SIM_ABI64 = 3 as per sysdeps/riscv/sgidefs.h.
* Comment out the store word instructions in pipe.S, as they're no
  longer needed.
* Fix sysdeps/unix/sysv/linux/riscv/bits/fcntl.h, mman.h, signum.h,
  and kernel_stat.h to match the ones provided in Linux.
* Remove sysdeps/unix/sysv/linux/riscv/xstatconv.c, rv64/fxstatat64.c,
  and bits/stat.h, to fall back on the ones already provided for
  all architectures.
* Negate the value created from the INTERNAL_SYSCALL_ERRNO macro
  to convert negative error values from the kernel.