[RISCV] Add support for new unprivileged extensions defined in profiles spec (#77458)

This adds minimal support for 7 new unprivileged extensions that were
defined as a part of
the RISC-V Profiles specification here:

https://github.com/riscv/riscv-profiles/blob/main/profiles.adoc#7-new-isa-extensions

* Ziccif: Main memory supports instruction fetch with atomicity
requirement
* Ziccrse: Main memory supports forward progress on LR/SC sequences
* Ziccamoa: Main memory supports all atomics in A
* Zicclsm: Main memory supports misaligned loads/stores
* Za64rs: Reservation set size of 64 bytes
* Za128rs: Reservation set size of 128 bytes
* Zic64b: Cache block size isf 64 bytes

As stated in the specification, these extensions don't add any new
features but
describe existing features. So this patch only adds parsing and
subtarget
features.