[openmp] Workaround bug in old Android 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.
Patch by Dan Albert <email@example.com>.
Test: Build, copied into the NDK, passed openmp test on ICS.