cmd/go: trim source paths when compiling C with -trimpath

When then go command is run with -trimpath, it will now use
-fdebug-prefix-map when invoking the C compiler (if supported) to
replace the source root directory with a dummy root directory.

This should prevent source directories from appearing either literally
or in compressed DWARF in linked binaries.

Updates #36072

Change-Id: Iedd08d5e886f81e981f11248a1be4ed4f58bdd29
Reviewed-on: https://go-review.googlesource.com/c/go/+/212101


Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>