[llvm] [clang][AArch64] Add --print-supported-extensions support (PR #65466)

[David Spickett via llvm-commits]

DavidSpickett wrote:

I've generalised this a bit but mostly kept to what RISC-V did. If we think it's time to completely generalise it then I'll figure that out.

Mostly want to know people are on board with supporting this for AArch64 first.

https://github.com/llvm/llvm-project/pull/65466