cmd/compile: only support -race and -msan where they work

Consolidate decision about whether -race and -msan options are
supported in cmd/internal/sys. Use consolidated functions in
cmd/compile and cmd/go. Use a copy of them in cmd/dist; cmd/dist can't
import cmd/internal/sys because Go 1.4 doesn't have it.

Fixes #24315

Change-Id: I9cecaed4895eb1a2a49379b4848db40de66d32a9
Reviewed-on: https://go-review.googlesource.com/121816


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>