The clang lld linker is faster than even ld.gold. When the compiler supports -fuse-ld=lld we should prefer to use lld.

This won't have much of an effect with our waterfall builds because the toolchain v3 version of GCC doesn't support --fuse-ld=lld. But the toolchain clang does, so this will speed up any clang builds we do (like ASAN). It will also speed up local developer builds for developers who are using clang, or using a system GCC new enough to support the flag.