Fix march flag in unittest

Author: drexin

cmake/modules/AddSwiftUnittests.cmake

@@ -49,7 +49,7 @@ function(add_swift_unittest test_dirname)
     set_property(TARGET "${test_dirname}" APPEND PROPERTY LINK_LIBRARIES "log")
   elseif("${CMAKE_SYSTEM_NAME}" STREQUAL "Linux")
     set_property(TARGET "${test_dirname}" APPEND PROPERTY COMPILE_FLAGS
-      "-march=core2")
+      "-Xcc -march=core2")
   endif()
 
   find_program(LDLLD_PATH "ld.lld")