Don't add -arch explictly to stdlib compile flags on Darwin

Rely instead on setting the `OSX_ARCHITECTURES` property (swiftlang#38415)

Address rdar://96087734

Author: edymtt

stdlib/cmake/modules/AddSwiftStdlib.cmake

@@ -104,7 +104,6 @@ function(_add_target_variant_c_compile_link_flags)
     # of options by target_compile_options -- this way no undesired
     # side effects are introduced should a new search path be added.
     list(APPEND result
-      "-arch" "${CFLAGS_ARCH}"
       "-F${SWIFT_SDK_${CFLAGS_SDK}_PATH}/../../../Developer/Library/Frameworks")
   endif()