cmd/cgo: add -trimpath flag allowing paths to be rewritten in outputs

cmd/cgo now has a -trimpath flag that behaves the same as the
-trimpath flag to cmd/compile. This will be used to correct paths
to cgo files that are overlaid.

The code that processes trimpath in internal/objapi has been slightly
refactored because it's currently only accessible via AbsFile, which
does some additional processing to the path names. Now an
ApplyRewrites function is exported that just applies the trimpath
rewrites.

Also remove unused srcfile argument to cmd/cgo.(*Package).godefs.

For #39958

Change-Id: I497d48d0bc2fe1f6ab2b5835cbe79f15b839ee59
Reviewed-on: https://go-review.googlesource.com/c/go/+/266358


Trust: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>