[llvm] [AArch64][BOLT] Document AArch64 optimization flag support (PR #198282)

[Paschalis Mpeis via llvm-commits]

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+=====================================
+AArch64 Optimization and Flags Status
+=====================================
+
+Overview
+--------
+
+This page summarizes default-off BOLT optimization flags that users may
+explicitly enable when optimizing AArch64 binaries.
+
+Ideally, BOLT is to be used with binaries linked with
+relocations (``--emit-relocs`` or ``-Wl,-q``) and representative profile data.
+
+Supported Flags
+---------------
+The following flags are supported for AArch64.
+
----------------
paschalis-mpeis wrote:

Not sure if it's worth to add the main reordering flags before any other ones?

I think split-functions could be put there as well as it's one of the main reordering optimizations we do, but we have to somehow list the `cdsplit` strategy under limitations.

https://github.com/llvm/llvm-project/pull/198282