ISPC: CompilerLauncher tests work properly with x86 builds

Tests/RunCMake/CompilerLauncher/ISPC-common.cmake

@@ -1,4 +1,8 @@
 enable_language(ISPC)
 enable_language(CXX)
 set(CMAKE_VERBOSE_MAKEFILE TRUE)
+
+if(CMAKE_SIZEOF_VOID_P EQUAL 4)
+  set(CMAKE_ISPC_FLAGS "--arch=x86")
+endif()
 add_executable(main main.cxx test.ispc)