Fix lld tests with LLVM_ENABLE_THREADS disabled.

With LLVM_ENABLE_THREADS disabled, all the llvm code assumes that it runs on
a single thread and doesn't use any mutexes.  lld still spawned lots of threads
in that case and called into llvm, assuming that llvm is thread-safe.

As fix, let lld use only a single thread if LLVM_ENABLE_THREADS is disabled.
I left in all the mutexes in lld. That means lld is a bit slower than
necessary in single-thread mode, but that's probably worth the simpler code.

llvm-svn: 242004