all: avoid use of cmd/compile -G flag in tests

The next CL will remove the -G flag, effectively hard-coding it to its
current default (-G=3).

Change-Id: Ib4743b529206928f9f1cca9fdb19989728327831
Reviewed-on: https://go-review.googlesource.com/c/go/+/388534


Reviewed-by: Keith Randall <khr@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>