libpmcstat: Fix a few ARM-specific issues with function symbols.

[jhb]

Summary

• Refine the checks for ARM mapping symbols and apply them on arm64 as well as 32-bit arm. In particular, mapping symbols can have additional characters and are not strictly limited to just "$a" but can append additional characters (e.g. "$a.1"). Add "$x" to the list of mapping symbol prefixes.

• Clear the LSB of function symbol addresses. Thumb function addresses set the LSB to enable Thumb mode. However, the actual function starts at the aligned address with LSB clear. Not clearing the LSB can cause pmcannotate to pass misaligned addresses to objdump when extracting disassembly.

Obtained from: CheriBSD
Sponsored by: University of Cambridge, Google, Inc.

[jhb]

I actually tested this on Morello which also sets the LSB to enable capability decoding mode similar to Thumb rather than Thumb. Morello also adds an additional type of mapping symbol ($c.<mumble>).

[emaste]

Looks reasonable to me