…system paths when -trimpath is set

CGO flag variables often include system paths for header files and
compiled libraries. The point of -trimpath is to avoid dependending on
system paths, so stamping these variables is counterproductive.

Moreover, the point of stamping build information is to improve
reproducibility. Since we don't also stamp the versions of C
compilers, headers, and libraries used in a cgo build, only the most
trivial cgo programs can be faithfully reproduced from the stamped
information.

Likewise, the -ldflags flag may include system-specific paths,
particularly if external linking is in use. For now, we omit -ldflags
entirely; however, in the future we may instead want to parse and
redact the individual flags.

Updates #52372.
Fixes #53119.

Change-Id: I73318a01cce4371d66955b3261fc7ee58d4b33dd
Reviewed-on: https://go-review.googlesource.com/c/go/+/409174
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
(cherry picked from commit a6e5be0)
Reviewed-on: https://go-review.googlesource.com/c/go/+/414794
Reviewed-by: Nooras Saba‎ <saba@golang.org>