runtime: move some malloc constants to internal/runtime/gc

These constants are needed by some future generator programs.

Change-Id: I5dccd009cbb3b2f321523bc0d8eaeb4c82e5df81
Reviewed-on: https://go-review.googlesource.com/c/go/+/655276


Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>