[llvm-branch-commits] [RISCV][lld] Support merging RISC-V Atomics ABI attributes (PR #97347)

[Paul Kirth via llvm-branch-commits]

https://github.com/ilovepi created https://github.com/llvm/llvm-project/pull/97347

This patch adds support for merging the atomic_abi attribute, specifid in
https://github.com/riscv-non-isa/riscv-elf-psabi-doc/blob/master/riscv-elf.adoc#tag_riscv_atomic_abi-14-uleb128version,
to LLD.

The atomics_abi tag merging is conducted as follows:

UNKNOWN is safe to merge with all other values.
A6C is compatible with A6S, and results in the A6C ABI.
A6C is incompatible with A7, and results in an error.
A6S and A7 are compatible, and merging results in the A7 ABI.
Note: the A7 is not yet supported in either LLVM or in any current hardware,
and is therefore omitted from attribute generation in RISCVTargetStreamer.

LLD support was split from https://github.com/llvm/llvm-project/pull/90266