[MIPS] Drop __ARCH_WANT_SYS_FADVISE64

sys_fadvise64() is not used on MIPS.  The libc can implement
both posix_fadvise() and posix_fadvise64() using sys_fadvise64_64().

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>