cmd/cgo: pass explicit -O0 to the compiler

The current implementation removes all of the optimization flags from
the compiler.
Added the -O0 optimization flag after the removal loop, so go can
compile cgo on every OS consistently.

Fixes #26487

Change-Id: Ia98bca90def186dfe10f50b1787c2f40d85533da
Reviewed-on: https://go-review.googlesource.com/127755


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