bionic: Add flag to restore legacy mmap behavior

* Pre-lollipop mmap would not care whether offset was signed
  or unsigned.
* Lollipop adds 64-bit support which results in sign extension
  of offset, causing a negative offset when
  a positive offset > 2^31 is given.

Change-Id: I5d19d898fc131cf848217974915d1b466a474f99