[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 <danalbert@google.com>.

Test: Build, copied into the NDK, passed openmp test on ICS.
Bug: https://github.com/android-ndk/ndk/issues/9
llvm-svn: 367070