[llvm] [clang] [RISCV][Clang][TargetParser] Support getting feature unaligned-scalar-mem from mcpu. (PR #71513)

[Francesco Petrogalli via cfe-commits]

https://github.com/fpetrogalli approved this pull request.

LGTM, with a couple of minor nits.

Thanks!

https://github.com/llvm/llvm-project/pull/71513