Fix test for memory allocators that want to look backwards.

Fix test for memory allocators that want to look backwards.

Summary:
- The NeedleFinderTest.NoSegFault test uses mprotect(2) to ensure that
our needle implementations don't peek past the end of memory. However,
some memory allocators (e.g. the defaulton OS X) seem to look at the
last byte of a page. As a result, if we allocate 2 pages and mark the
second as PROT_NONE, then ask the memory allocator for 2 more pages,
it may end up peeking at the last byte of the second page, triggering
a SIGBUS. To work around this, allocate 8 pages of memory and only
mark the second as PROT_NONE.
- Clear mprotect(2) bits before freeing memory.

Test Plan:
- fbconfig -r folly && fbmake runtests
- ./configure && make check on Ubuntu/FC/Mac

Reviewed By: delong.j@fb.com

FB internal diff: D998592