[release-branch.go1.14-security] cmd/go: in cgoflags, permit -DX1, prohibit -Wp,-D,opt

Restrict -D and -U to ASCII C identifiers, but do permit trailing digits.
When using -Wp, prohibit commas in -D values.

Thanks to Imre Rad (https://www.linkedin.com/in/imre-rad-2358749b) for reporting this.

Fixes CVE-2020-28367

Change-Id: Ibfc4dfdd6e6c258e131448e7682610c44eee9492
Reviewed-on: https://go-review.googlesource.com/c/go/+/267277


Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/899923


Reviewed-by: Filippo Valsorda <valsorda@google.com>