tsan: reduce internal allocator region from 16M to 1M
16M regions can waste almost 1G for nothing.
Since region size is used only during initial heap growth,
it's unclear why we even need such huge regions.



git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@214027 91177308-0d34-0410-b5e6-96231b3b80d8