Most of the portability problems for retargetting to another little endian 64-bit architecture aren't the easy ones of bit widths and integer types, but rather more subtle things like a more relaxed memory model and different performance characteristics.

It's not worth the overhead of attempting to ship bitcode. Apple has shipped products on 6502, 68000, PPC, x86, x86-64, ARM, and ARM64. The transitions have been annoying for the lost software, but that's not an unusual occurrence for an OSX upgrade either.