Remove internal usage of extensible stat functions

It replaces the internal usage of __{f,l}xstat{at}{64} with the
__{f,l}stat{at}{64}.  It should not change the generate code since
sys/stat.h explicit defines redirections to internal calls back to
xstat* symbols.

Checked with a build for all affected ABIs.  I also check on
x86_64-linux-gnu and i686-linux-gnu.

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