cmd/cgo: don't update each call in place

cmd/cgo: don't update each call in place

Updating each call in place broke when there were multiple cgo calls
used as arguments to another cgo call where some required rewriting.
Instead, rewrite calls to strings via the existing mangling mechanism,
and only substitute the top level call in place.

Fixes #28540

Change-Id: Ifd66f04c205adc4ad6dd5ee8e79e57dce17e86bb
Reviewed-on: https://go-review.googlesource.com/c/146860
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>