[WebAssembly] Force use of lld for test/Driver/wasm-toolchain.c(pp)

lld is the only supported linker that works for WebAssembly, so ensure
clang is using it for this test. This gets the tests passing when
configuring clang to use a different linker by default.

Differential Revision: https://reviews.llvm.org/D49897

llvm-svn: 339158