[gn build] Use LLD as host linker by default on macOS if clang_base_path is set

lld/mac should be stable enough to use it as host linker. I've been
using `use_lld=true` in my local args.gn for many months now and it
works fine (and links much faster than ld64).

Differential Revision: https://reviews.llvm.org/D112622