cmd/cgo: remove -O options when generating compiler errors
The cgo tool generates compiler errors to find out what kind of name it
is using. Turning on optimization can confuse that process by producing
new unexpected messages.
Fixes #14669.
Change-Id: Idc8e35fd259711ecc9638566b691c11d17140325
Reviewed-on: https://go-review.googlesource.com/23231
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Russ Cox <rsc@golang.org>