The addressing mode of global variable was missing, whereas the
compiler may make use of it, causing "illegal combination" error.
This CL adds support of that addressing mode.

Fixes #22390.

Change-Id: Ic8eade31aba73e6fb895f758ee7f277f8f1832ef
Reviewed-on: https://go-review.googlesource.com/72610
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>