Revert r257686 "With COMPILER_RT_INCLUDE_TESTS turned ON and in a cross compiling"

This broke the build. For example, from
http://lab.llvm.org:8011/builders/clang-cmake-aarch64-full/builds/1191/steps/cmake%20stage%201/logs/stdio:

	-- Compiler-RT supported architectures: aarch64
	CMake Error at projects/compiler-rt/cmake/Modules/AddCompilerRT.cmake:170 (string):
		string sub-command REPLACE requires at least four arguments.
	Call Stack (most recent call first):
		projects/compiler-rt/lib/CMakeLists.txt:4 (include)

llvm-svn: 257694