linux: Consolidate fstatvfs implementations

There is no need to handle ENOSYS on fstatfs64 call, required only
for alpha (where is already fallbacks to fstatfs).  The wordsize
internal_statvfs64.c is removed, since how the LFS support is
provided by fstatvfs64.c (used on 64-bit architectures as well).

Checked on x86_64-linux-gnu.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>