cmd/internal/obj/x86: handle global reference in From3 in dynlink mode

cmd/internal/obj/x86: handle global reference in From3 in dynlink mode

In dynlink mode, we rewrite reference to a global variable to
a load from the GOT. Currently this code does not handle the case
that the global reference is in From3 of a Prog. Most instructions
don't expect a memory operand in From3, but some do, like
VGF2P8AFFINEQB. Handle this case.

Change-Id: Ibb6773606e6967bcc629d9ef5dac6e050f4008ef
Reviewed-on: https://go-review.googlesource.com/c/go/+/722181
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>