[WebAssembly] Fix location and -flavor when running lld

Add the toolchain installation directory to the program
path so that lld can be found.

Change -flavor to wasm.  Although this new flavor hasn't
yet landed in upstream lld yet there are no point in
passing wasm objects the gnu flavor.

Differential Revision: https://reviews.llvm.org/D32976

llvm-svn: 302558