support: Use macros for *stat wrappers

Macros will automatically use the correct types, without
having to fiddle with internal glibc macros.  It's also
impossible to get the types wrong due to aliasing because
support_check_stat_fd and support_check_stat_path do not
depend on the struct stat* types.

The changes reveal some inconsistencies in tests.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>