tsan: support mmap(MAP_32BIT)
Allow user memory in the first TB of address space.
This also enabled non-pie binaries and freebsd.
Fixes issue:
https://code.google.com/p/thread-sanitizer/issues/detail?id=5



git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@220571 91177308-0d34-0410-b5e6-96231b3b80d8