elf_common.h: Add STO_AARCH64_VARIANT_PCS

[andrew]

Summary

This is used to mark symbols that use a variant procedure call with
a different calling convention to the main ABI.

Sponsored by: Arm Ltd

[emaste]

Do we need to add support to e.g. ELF Tool Chain readelf and nm to decode these values?

[kib]

Is it flag or enumeration?

[andrew]

A flag. https://github.com/ARM-software/abi-aa/blob/main/aaelf64/aaelf64.rst#st_other-values lists it under Processor specific st_other flags.

[andrew]

In D44868#1022920, @emaste wrote:

Do we need to add support to e.g. ELF Tool Chain readelf and nm to decode these values?

It looks like binutils adds [VARIANT_PCS] to symbols with this flag, e.g.

97: 0000000000000000 0 FUNC GLOBAL DEFAULT [VARIANT_PCS] UND variant_pcs_test