linux: Move {f}xstat{at} to compat symbols

They are no interna uses anymore.  The riscv32 ABI was added on 2.33,
so it is safe to remove the old __{f,l}stat{at} symbols and just
provide the newer {f,l}stat{at} ones.

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>