build: fix darwin/arm broken on macOS 10.12 with Xcode 8.0

Xcode 8.0 has been donen't support the iOS 5 anymore

Fixes #18390.

Change-Id: Icc97e09424780c610a8fe173d0cf461d76b06da4
Reviewed-on: https://go-review.googlesource.com/34673
Reviewed-by: David Crawshaw <crawshaw@golang.org>

misc/ios/clangwrap.sh

@@ -17,4 +17,4 @@ else
 	exit 1
 fi
 
-exec $CLANG -arch $CLANGARCH -isysroot $SDK_PATH "$@"
+exec $CLANG -arch $CLANGARCH -isysroot $SDK_PATH -mios-version-min=6.0 "$@"