cmd/go: support cgo files in overlays

This is a roll-forward of golang.org/cl/267197, which was reverted in
golang.org/cl/267357. It makes the following changes in addition to
the ones in the next paragraph: It avoids outputting trimpath
arguments for an overlay unless the overlay affects the package being
compiled (to avoid hitting windows command line argument limits), and
it fixes processing of regexps in the script test framework to treat
the first *non flag* argument to grep, stdout, and stderr as a regexp,
not just the first argument.

golang.org/cl/267917 was a roll-forward of golang.org/cl/262618, which
was reverted in golang.org/cl/267037. The only differences between
this CL and the original were the three calls to fflush from the C
files in build_overlay.txt, to guarantee that the string we were
expecting was
actually written out.

The CL requires rewriting the paths of the files passed to the cgo
tool toolchain to use the overlaid paths instead of the disk paths of
files. Because the directories of the overlaid paths don't exist in
general, the cgo tool have been updated to run in base.Cwd instead of
the package directory.

For #39958

Change-Id: I1bd96db257564bcfd95b3502aeca14d04bd28618
Reviewed-on: https://go-review.googlesource.com/c/go/+/267797


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