ac/radv: move llvm compiler info to struct and init in one place

ac/radv: move llvm compiler info to struct and init in one place

This ports radv to the shared code, however due to a bug in LLVM
version prior to 7, radv cannot add target info at this stage,
as it would leak one for every shader compile, however I'd prefer
to keep this llvm damage in the shared code, since it isn't the
driver at fault here. We just add a flag to denote if the driver
can support leaking the target info or not, and the common code
does the right thing depending on the llvm version.

 Reviewed-by: Marek Olšák <marek.olsak@amd.com>