[llvm] [Docs][llvm-exegesis] Clarify AArch64 support (PR #114989)

[Aiden Grossman via llvm-commits]

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@@ -28,7 +36,7 @@ architectures:
   * 64-bit only due to this being the only implemented calling convention
     in `llvm-exegesis` currently.
 * ARM
-  * AArch64 only
+  * Very experimental AArch64 support only, most opcodes probably won't work.
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boomanaiden154 wrote:

Same clarification here.

https://github.com/llvm/llvm-project/pull/114989