linux: Fix fstat64 on alpha and sparc64

The 551101e8 change is incorrect for
alpha and sparc, since __NR_stat is defined by both kABI.  Use
__NR_newfstat to check whether to fallback to __NR_fstat64 (similar
to what fstatat64 does).

Checked on sparc64-linux-gnu.
Reviewed-by: Carlos O'Donell <carlos@redhat.com>