gcc: Various fixups to ensure consistent gcc builds

(ported from oe-core for consistency)

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 in oe-core where the interpretor could
become corrupted. This was due to the sed expression corrupting
the source directory.

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>