compiler_wrapper: refactor goma flag parsing

This is in preparation for adding more flag parsing of this type.

BUG=b:190741226
TEST=go test

Change-Id: Ief431a6e30b6ba22767cdd46247e362508addd4b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/2956694
Reviewed-by: Ryan Beltran <ryanbeltran@chromium.org>
Tested-by: George Burgess <gbiv@chromium.org>
2 files changed