[compiler-rt] Fix cross-compiling detection

Previously the detection compares "CMAKE_SYSTEM_NAME" and
"CMAKE_TARGET_SYSTEM_NAME", which only contains OS names like "Linux".
This is not sufficient when cross-compiling for different target CPUs.

This patch uses CMAKE_CROSSCOMPILING, which sets automatically by CMake
when cross compiling.

Reviewed By: smeenai, phosek

Differential Revision: https://reviews.llvm.org/D147103

compiler-rt/CMakeLists.txt

@@ -162,7 +162,7 @@ pythonize_bool(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR)
 # We support running instrumented tests when we're not cross-compiling
 # and target a UNIX-like system or Windows.
 # We can run tests on Android even when we are cross-compiling.
-if(("${CMAKE_HOST_SYSTEM_NAME}" STREQUAL "${CMAKE_SYSTEM_NAME}" AND (UNIX OR WIN32))
+if((NOT "${CMAKE_CROSSCOMPILING}" AND (UNIX OR WIN32))
    OR ANDROID OR COMPILER_RT_EMULATOR)
   option(COMPILER_RT_CAN_EXECUTE_TESTS "Can we execute instrumented tests" ON)
 else()