syscall, internal/syscall/unix: fix fstatat on linux/mips64
On linux/mips64, the syscall.Stat_t struct does not match the
kernel version of the struct. Functions that operate on a Stat_t
translate between it and the kernel struct.
The fstatat function was not doing this translation.
Make it do so.
Export a syscall.Fstatat on mips64 for usage by
internal/syscall/unix. Perhaps we should just do this on all
architectures, but this is the smaller change for now.
Fixes #70659
Change-Id: I38e36473689be25861953b418c9abc5b270a7bcf
Reviewed-on: https://go-review.googlesource.com/c/go/+/633280
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Ian Lance Taylor <iant@google.com>