Fix the cross compilation of unit tests. NFC (second attempt)

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)

Differential Revision: http://reviews.llvm.org/D16165

llvm-svn: 257783