stand: aarch64 has different nlinks than amd64

Some typedefs are system dependent, so move them into stat_arch.h where
they are used. On amd64, nlinks is a int64_t, while on aarch64 it's an
int (or int32_t).

Sponsored by: Netflix

(cherry picked from commit 929c6216b1668b9a8bc7d9d9edfd528ef74c69f8)