LLVM: fix universal compilation and allow for universal + shared

Two minor changes to the formula here.  First a flag is added to change the environment to std in the case of building a universal binary.  As seen in #17352 there is a problem with SUPERENV stripping universal compilation flags.

Secondly I have removed the check that prevented compiling universal + shared at the same time.  While this might have been broken in the past I have successfully compiled with both flags.

Finally I was curious if there is any way to roll these two changes back to the archived 3.1 formula, as it currently is not possible to compile a universal binary without adding `--env=std` flag