Darwin - fix intermitent crashes in import crashinfo

    
Summary:
"import crashinfo" was probably failing because multiple dotest invocations
are all trying to compile crashinfo.so at the same time.
    
I've put a mutex around the compile step to prevent this.
   
Reviewers: clayborg, chying
    
Subscribers: lldb-commits
    
Differential Revision: http://reviews.llvm.org/D9732

llvm-svn: 237230