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/syscall_darwin_libSystem.go

@@ -0,0 +1,31 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build darwin,go1.12
+
+package unix
+
+import "unsafe"
+
+// Implemented in the runtime package (runtime/sys_darwin.go)
+func syscall_syscall(fn, a1, a2, a3 uintptr) (r1, r2 uintptr, err Errno)
+func syscall_syscall6(fn, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2 uintptr, err Errno)
+func syscall_syscall6X(fn, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2 uintptr, err Errno)
+func syscall_syscall9(fn, a1, a2, a3, a4, a5, a6, a7, a8, a9 uintptr) (r1, r2 uintptr, err Errno) // 32-bit only
+func syscall_rawSyscall(fn, a1, a2, a3 uintptr) (r1, r2 uintptr, err Errno)
+func syscall_rawSyscall6(fn, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2 uintptr, err Errno)
+
+//go:linkname syscall_syscall syscall.syscall
+//go:linkname syscall_syscall6 syscall.syscall6
+//go:linkname syscall_syscall6X syscall.syscall6X
+//go:linkname syscall_syscall9 syscall.syscall9
+//go:linkname syscall_rawSyscall syscall.rawSyscall
+//go:linkname syscall_rawSyscall6 syscall.rawSyscall6
+
+// Find the entry point for f. See comments in runtime/proc.go for the
+// function of the same name.
+//go:nosplit
+func funcPC(f func()) uintptr {
+	return **(**uintptr)(unsafe.Pointer(&f))
+}