Workaround bug in old pthread_attr_setstacksize.

Round the stack size to a multiple of the page size. Older versions of
Android (until KitKat) would fail pthread_attr_setstacksize with
EINVAL if the stack size was not a multiple of the page size.

Test: Build, copied into the NDK, passed openmp test on ICS.
Bug: https://github.com/android-ndk/ndk/issues/9
Change-Id: I30f171d33f0eb3547fe37b8770c60a0b1025717c