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

[Aiden Grossman via llvm-commits]

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@@ -13,8 +13,16 @@ Analysis mode in `llvm-exegesis` is supported on all platforms on which LLVM is.
 #### Currently Supported Operating Systems for Benchmarking
 
 Currently, `llvm-exegesis`  only supports benchmarking on Linux. This is mainly
-due to a dependency on the Linux perf subsystem for reading performance
-counters.
+due to a dependency on the Linux perf subsystem and libpfm4 for reading
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boomanaiden154 wrote:

This should probably go in the LLVM CMake documentation. This page isn't intended to provide detailed build directions, and people are more likely to look there.

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