pk: thwart an attempt from the compiler to optimize

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!