For the new ar --plugin libLLVMgold.so to work we need libLTO linked with

pthreads, otherwise ar complains of unresolved references to pthread_mutex_*.
LTO doesn't actually use pthreads, but ManagedStatic does.

Fix this by linking in LIBS (that contains pthreads) for LTO and gold.
For now this links in more libs than needed (libffi for example), we can figure
out later how to link in those libs per-tool later.

llvm-svn: 72430