The WebAssembly target itself is similar to the X86 target in https://reviews.llvm.org/rL348903
The unittests bits are similar to the corresponding AArch64 in https://reviews.llvm.org/rL350499
The motivation for this target is solely that it has a unit test and I want to enable the GN<->CMake unittest syncing check for llvm. (After this, only the PowerPC target is needed and I can turn it on.)