unix: reverse default for 1.11/1.12 split

Make 1.11 the explicitly named case, and 1.12 the default.

Change-Id: Ic3674860eb9e8a5bb1cf1da12c5ac583cc0b0633
Reviewed-on: https://go-review.googlesource.com/c/155397
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/darwin_test.go

@@ -69,7 +69,7 @@ func init() {
 	}
 }
 
-// All the _trampoline functions in zsyscall_darwin_$ARCH.1_12.s
+// All the _trampoline functions in zsyscall_darwin_$ARCH.s
 var darwinTests = [...]darwinTest{
 	{"getgroups", libc_getgroups_trampoline},
 	{"setgroups", libc_setgroups_trampoline},