[RISCV] Support RISCV Atomics ABI attributes (#84597)

This patch adds support for 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.

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 ommited from attribute generation in
RISCVTargetStreamer.