[clang] [RISCV] Support predefined marcro __riscv_misaligned_{fast,avoid}. (PR #65756)

[Fangrui Song via cfe-commits]

MaskRay wrote:

> [RISCV] Support predefined marcro __riscv_misaligned_{fast,avoid}.

The majority of patches don't add a trailing period in the title.

https://github.com/llvm/llvm-project/pull/65756