gcc: Resolve relative prefix-map filenames

gcc: Resolve relative prefix-map filenames

Add a patch to gcc so that relative paths are handled by -fdebug-prefix-map
and friends. In OE we use relative paths in autotools and removing that
creates a lot of issues we'd have to fix. This alternative allows us to
fix the paths within gcc and improve our debug file coverage (and SPDX
manifests) accordingly.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>