[WebAssembly] Remove duplicated RTLIB names

Remove the tight coupling between llvm/CodeGenRuntimeLibcalls.def and
the table of supported singatures for wasm. This will allow adding new libcalls
without changing wasm's signature table.

Also, some cleanup:
Use ManagedStatics instead of const tables to avoid memory/binary bloat.
Use a StringMap instead of a linear search for name lookup.

Differential Revision: https://reviews.llvm.org/D35592

llvm-svn: 322802