syscall, internal/syscall/unix: add Openat support for wasip1
The syscall package is mostly frozen, but wasip1 file syscall
support was added to syscall and the Open and Openat
implementations overlap. Implement Openat in syscall for
overall simplicity.
We already have syscall.Openat for some platforms, so this
doesn't add any new functions to syscall.
For #67002
Change-Id: Ia34b12ef11fc7a3b7832e07b3546a760c23efe5b
Reviewed-on: https://go-review.googlesource.com/c/go/+/617378
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Ian Lance Taylor <iant@golang.org> Reviewed-by: Cherry Mui <cherryyz@google.com>