> There are LLVM targets with corresponding ABIs that are nonspecific to architectures like PNaCL but Apple is not using them

They do indeed support LLVM IR target(s) that are, in fact, much more architecturally-nonspecific than PNaCl, namely the spir[64]-unknown-unknown target. See https://github.com/KhronosGroup/SPIR and /System/Library/Frameworks/OpenCL.framework/Versions/A/Libraries/openclc (on a Mac, since 10.7).

I don't know if bitcode with this target triple is necessarily suitable for the Apple Watch, however.