[clang] [llvm] [AArch64][RISCV] Improve the tests for --print-enabled-extensions and --print-supported-extensions (PR #97829)

[Jon Roelofs via cfe-commits]

================
@@ -0,0 +1,24 @@
+// REQUIRES: aarch64-registered-target
----------------
jroelofs wrote:

these should probably all go in an `aarch64` folder with a `lit.local.cfg`:

```
targets = set(config.root.targets_to_build.split())
if not "AArch64" in targets:
    config.unsupported = True
```

and then drop the `REQUIRES:` lines from them.

https://github.com/llvm/llvm-project/pull/97829