We already remove -rdynamic if -static appears in -extldflags.
Extend that to apply to CGO_LDFLAGS and #cgo LDFLAGS as well.

Updates #26197

Change-Id: Ibb62d1b20726916a12fd889acb05c1c559a5ace2
Reviewed-on: https://go-review.googlesource.com/122135
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>