tsan: fix crash when a program registers zillions of atexit callbacks

I don't remember that crash on mmap in internal allocator
ever yielded anything useful, only crashes in rare wierd untested situations.
One of the reasons for crash was to catch if tsan starts allocating
clocks using mmap. Tsan does not allocate clocks using internal_alloc anymore.
Solve it once and for all by allowing mmaps.

llvm-svn: 217929