Initial support to bytetype

[gulugulubing]

This is relate to #4982.
ABI of aarch64 is simpler than x86-64(and I only have a macbook), so I just implement byte type on aarch64 first.

Draft: Experimental b8 interop for 8-bit parameters

Status

• AArch64 ABI rewrite (ubyte/char → b8 + zeroext)
• Call-site: bitcast i8 → b8 (piggybacking the original addExplicitArguments)
• Return-site: bitcast b8 → i8
• Callee entry: bitcast b8 → i8
• Callee return: bitcast i8 → b8

Gate

-fc-interop-llvm-byte (default off)

Known Limitations

• LLVM upstream work for byte types is still in progress
• Clang does not yet emit b8 for unsigned char
• LTO testing uses hand-written .ll for C side

[thewilsonator]

Thanks for looking into this