[gn build] Add build files for llvm/lib/Target/WebAssembly + tests

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.)

Differential Revision: https://reviews.llvm.org/D56374

llvm-svn: 350628