runtime: use the immortal weak handle map for sbrk mode

runtime: use the immortal weak handle map for sbrk mode

Currently weak pointers break in sbrk mode. We can just use the immortal
weak handle map for weak pointers in this case, since nothing is ever
freed.

Fixes #69729.

Change-Id: Ie9fa7e203c22776dc9eb3601c6480107d9ad0c99
Reviewed-on: https://go-review.googlesource.com/c/go/+/674656
Reviewed-by: Carlos Amedee <carlos@golang.org>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
TryBot-Bypass: Michael Knyszek <mknyszek@google.com>