test/run: handle asmcheck -gcflags= commands better

The default is for later flags to override earlier ones,
so if the asmcheck set flags, it lost the important -S=2.

Change-Id: Id538254908d658da2acb55157ac4f6fa44f6a467
Reviewed-on: https://go-review.googlesource.com/c/go/+/222820


Reviewed-by: Keith Randall <khr@golang.org>