[RISCV] Support predefined macro __riscv_misaligned_[fast,avoid]. (#65756)

RISC-V C API introduced predefined macro to achieve hints about
unaligned accesses ([pr]). This patch defines __riscv_misaligned_fast
when using -mno-strict-align, otherwise, defines
__riscv_misaligned_avoid.

Note: This ignores __riscv_misaligned_slow which is also defined by
spec.

[pr]: https://github.com/riscv-non-isa/riscv-c-api-doc/pull/40