[clang] [clang] Better bitfield access units (PR #65742)

[YunQiang Su via cfe-commits]

wzssyqa wrote:

For MIPSr6, it is just like AARCH64, since some microarchitecture doesn't support mis-unaligned well in hardware level, so we need an options to disable it: kernel may need it.

For GCC, we have `-mno-unalgined-access`. We need also add this option to clang.

https://github.com/llvm/llvm-project/pull/65742