[WebAssembly] Make getArchTypePrefix return "wasm".
[WebAssembly] Make getArchTypePrefix return "wasm".

The arch prefix string isn't currently being used for anything on
WebAssembly, but if it were to be used, it makes sense to use the
same arch prefix string for wasm32 and wasm64.


git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@245252 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm