With COMPILER_RT_INCLUDE_TESTS turned ON and in a cross compiling
environment, the unit tests fail to link. This patch does the following changes
- Rename COMPILER_RT_TEST_CFLAGS to COMPILER_RT_UNITTEST_CFLAGS to reflect the way it's used.
- Add COMPILER_RT_TEST_COMPILER_CFLAGS to COMPILER_RT_UNITTEST_CFLAGS so that cross-compiler would be able to build/compile the unit tests
- Add COMPILER_RT_UNITTEST_LINKFLAGS to COMPILER_RT_UNITTEST_CFLAGS so that cross-compiler would be able to link the unit tests (if needed)