Moving these intrinsics to a base package enables other internal/runtime
packages to use them.
For #54766.
Change-Id: I0b3eded3bb45af53e3eb5bab93e3792e6a8beb46
Reviewed-on: https://go-review.googlesource.com/c/go/+/613260
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
