gcc: Add a gcc 12 snapshot

This commit is identical to what I have submitted upstream and what
has since been picked up by upstream devs.

It is not yet committed upstream because upstream would rather wait
for an official gcc 12-rc, but upstream's gcc 12 will have the same
patches when it arrives.

Signed-off-by: Bernhard Rosenkränzer <bernhard.rosenkraenzer.ext@huawei.com>