Revert "Use mmap to create the pthread_internal_t."

Unfortunately, this change provokes random crashes for ART, and
I have seen libc crashes on the device that might be related to it.

Reverting it fixes the ART crashes. there is unfortunately no
stack trace for the crashes, but just a "Segmentation fault" message.


This reverts commit cc5f6543.

Change-Id: I68dca8e1e9b9edcce7eb84596e8db619e40e8052