This adds a whitelisted subset of compiler flags to the DW_AT_producer
DWARF attribute of each package compilation unit DIE. This is common
practice in DWARF and can help debuggers determine the quality of the
produced debugging information.

Fixes #22168.

Change-Id: I1b994ef2262aa9b88b68eb6e883695d1103acc58
Reviewed-on: https://go-review.googlesource.com/71430Reviewed-by: Ian Lance Taylor <iant@golang.org>