[RISCV] Add a subtarget feature to enable unaligned scalar loads and stores

A RISCV implementation can choose to implement unaligned load/store support. We currently don't have a way for such a processor to indicate a preference for unaligned load/stores, so add a subtarget feature.

There doesn't appear to be a formal extension for unaligned support. The RISCV Profiles (https://github.com/riscv/riscv-profiles/blob/main/profiles.adoc#rva20u64-profile) docs use the name Zicclsm, but a) that doesn't appear to actually been standardized, and b) isn't quite what we want here anyway due to the perf comment.

Instead, we can follow precedent from other backends and have a feature flag for the existence of misaligned load/stores with sufficient performance that user code should actually use them.

Differential Revision: https://reviews.llvm.org/D126085