Add cxx backend?

[Gankra]

We have a "c"/"cc" backend. It would be nice to extend/generalize that to a cxx backend.

The C backend already has some cases for what compiler it's targetting, on paper there's very little to change for cxx, but I haven't thought it through.

abi-cafe/src/abis/c.rs

Lines 47 to 52 in 6ba6865

	pub struct CcAbiImpl {
	cc_flavor: CCFlavor,
	platform: Platform,
	mode: &'static str,
	}

The one thing that jumps to mind is enum class existing and having better repr support.