fix posix_fadvise syscall args on powerpc, unify with arm fix

fix posix_fadvise syscall args on powerpc, unify with arm fix

commit 6d38c9cf80f47623e5e48190046673bbd0dc410b provided an
arm-specific version of posix_fadvise to address the alternate
argument order the kernel expects on arm, but neglected to address
that powerpc (32-bit) has the same issue. instead of having arch
variant files in duplicate, simply put the alternate version in the
top-level file under the control of a macro defined in syscall_arch.h.