[RISCV][MC] MC layer support for the experimental zalasr extension (#79911)

This PR implements experimental support for the RISC-V Atomic
Load-Acquire and Store-Release Extension (Zalasr). It has been approved
to be pursued as a fast track extension
(https://lists.riscv.org/g/tech-unprivileged/topic/arc_architecture_review/101951698),
but has not yet been approved by ARC or ratified. See
https://github.com/mehnadnerd/riscv-zalasr

 for draft spec.

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Co-authored-by: brs <turtwig@utexas.edu>
Co-authored-by: Philip Reames <preames@rivosinc.com>