[MIPS] fstatat syscall names

MIPS is the only port to call its fstatat()-related syscalls
"__NR_fstatat".  Now I can see why that might be seen as every
other port being wrong, but I think for o32, it is at best confusing.
__NR_fstat provides a plain (32-bit) stat while __NR_fstatat provides a
64-bit stat.  Changing the name to __NR_fstatat64 would make things more
explicit, match x86, and make the glibc port slightly easier.

The current name is more appropriate for n32 and n64, but it would be
appropriate for other 64-bit targets too, and those targets have chosen
to call it __NR_newfstatat instead.  Using the same name for MIPS would
again be more consistent and make the glibc port slightly easier.

I'm not wedded to this idea if the current names are preferred,
but FWIW...

Signed-off-by: Richard Sandiford <richard@codesourcery.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>