Stop shadowing __pure2 in math.h.

I'd actually fixed sys/cdefs.h quite some time ago, but that
definition of __pure2 was being clobbered by the bogus one in math.h.

Test: clang -E -dM --sysroot $NDK/platforms/android-9/arch-arm/ \
          platforms/android-9/include/math.h | grep __pure2
Bug: http://b.android.com/62020

Change-Id: I9b93a7c46075a02ed1ea0ca2cc77624b15955c7f
3 files changed