Set march for unittest on Linux

Author: drexin

cmake/modules/AddSwiftUnittests.cmake

@@ -48,8 +48,8 @@ function(add_swift_unittest test_dirname)
       "${android_system_libs}")
     set_property(TARGET "${test_dirname}" APPEND PROPERTY LINK_LIBRARIES "log")
   elseif("${CMAKE_SYSTEM_NAME}" STREQUAL "Linux")
-    set_property(TARGET "${test_dirname}" APPEND PROPERTY LINK_LIBRARIES
-      "atomic")
+    set_property(TARGET "${test_dirname}" APPEND PROPERTY COMPILE_FLAGS
+      "-march=core2")
   endif()
 
   find_program(LDLLD_PATH "ld.lld")