cmd/cgo: don't pass CGO_CFLAGS -g options to debug info generation

Fixes #26144

Change-Id: Ie69dab1bd819eaf158be11769903b2636bbcf516
Reviewed-on: https://go-review.googlesource.com/c/152165


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Nikhil Benesch <nikhil.benesch@gmail.com>