This patch follows up my previous patch and supports more variants of 
LLVM 12.

There are still other incompatibilities with LLVM 12, but this at least 
the ELF attributes should now automatically tune to any LLVM 9, 10, or 
12 assembler (It would be nice if one set of options would just work 
everywhere, but no).

LLVM 11 was not tested, but is broken in other ways in any case. LLVM 13 
(dev) needs more work.

Unfortunately, the need for configure tests and the CLI instability 
within the LLVM 12 release branch means that GCC probably needs to be 
rebuilt whenever LLVM is upgraded, even for minor versions.

Andrew