[clang] [flang] [llvm] [AArch64] Add ability to list extensions enabled for a target (PR #95805)

[Lucas Duarte Prates via cfe-commits]

pratlucas wrote:

> Looks like the new tests in `Driver` are missing `// REQUIRES: aarch64-registered-target`

Yes, I'm adding those now. There was also an odd ThreadSanitizer failure in clangd (https://lab.llvm.org/buildbot/#/builders/134/builds/680), which I'm investigating to check if is indeed related to this change.

https://github.com/llvm/llvm-project/pull/95805