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>

src/cmd/cgo/gcc.go

@@ -1693,6 +1693,9 @@ func (p *Package) gccErrors(stdin []byte) string {
 		}
 	}
 
+	// Force -O0 optimization
+	nargs = append(nargs, "-O0")
+
 	if *debugGcc {
 		fmt.Fprintf(os.Stderr, "$ %s <<EOF\n", strings.Join(nargs, " "))
 		os.Stderr.Write(stdin)