cmd/go: correctly report that -msan needs CGO_ENABLED=1

cmd/go: correctly report that -msan needs CGO_ENABLED=1

Previously, if CGO_ENABLED=0 was set when building
with -msan, the error message printed was:

  -race requires cgo; enable cgo by setting CGO_ENABLED=1

yet the instrumentation flag passed in was -msan. This CL
fixes that message to correctly report that -msan needed
CGO_ENABLED=1, and likewise if -race, report -race needed it.

Fixes #21895

Change-Id: If423d520daae7847fb38cc97c3192ada5d960f9d
Reviewed-on: https://go-review.googlesource.com/63930
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>