gcc: Various fixups to ensure consistent gcc builds

gcc: Various fixups to ensure consistent gcc builds

We ensure that:
* the shared work directory contains PR and ensure PR values are consistent across gcc builds
* the regexp to handle library directories is in a specific task and run once

This avoids breakage that was seen in incremental builds after commit
be1f70d68b6b75772ebab8bdff683ddd7c42b0cd where the interpretor could
become corrupted. This was due to the sed expression corrupting
the source directory.

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