Use wasm-ld instead of "lld -flavor wasm".

Invoking lld as ld.lld, ld.ld64, lld-link or wasm-ld is preferred
than invoking lld as lld and pass an -flavor option. We have "lld"
file mostly for historical reasons.

Differential Revision: https://reviews.llvm.org/D43407

llvm-svn: 325405