Make it possible to choose llvm toolchain to come from local system

Currently, the pre-packaged toolchain does not work on all systems, for instance @cdleary has to remove it to compile xls locally on his machine:

* https://github.com/google/xls/commit/76d4ad5524dc5207f45c85f1498c445ffb2d6b99
* Also @proppy had to do that to compile on the Mac: https://github.com/google/xls/pull/1570#issuecomment-2320601018

If there is a sufficiently compatible toolchain on the system (like in the cases above), it would be good if these users can opt out without having to patch out the toolchain first. Maybe we need to define an option in bazael e.g. `//xls_use_system_compiler` that we then can select on to not load our toolchain. Not sure how that would be done, but it is probably is not terribly complicated either.
Should be tackled once #1570 is in.