Allow 0 in math functions

Allow non-negative integers instead of positive integers in math
functions so that LOCAL_RENDERSCRIPT_TARGET_API := 0 can be compared
against 21.

Test: make -f ${ANDROID_BUILD_TOP}/build/make/core/math.mk RUN_MATH_TESTS=true
Test: $(get_build_var CKATI) -f ${ANDROID_BUILD_TOP}/build/make/core/math.mk RUN_MATH_TESTS=true
Change-Id: If2504b7de81f8dd676af749a04c9f3e580f298b6