linux: Implement {l}fstat{at} in terms of fstatat

Both fstatat and fstata64 calls the old fxstatat and fxstatat64
repectivelly with _STAT_VER, the one currently exported as default
for all ABIs.

Checked with a build for all affected ABIs. I also checked on x86_64,
i686, powerpc, powerpc64le, sparcv9, sparc64, s390, and s390x.

Reviewed-by: Lukasz Majewski <lukma@denx.de>