buffer: split runtime.s into pre- and post-Go 1.8 files.

Go 1.8 renamed this function; cf. golang/go@87e48c5.

internal/buffer/runtime.go

@@ -19,6 +19,8 @@ import "unsafe"
 //go:noescape
 
 // Zero the n bytes starting at p.
+//
+// REQUIRES: the region does not contain any Go pointers.
 func memclr(p unsafe.Pointer, n uintptr)
 
 //go:noescape

internal/buffer/runtime_go1.8.s

@@ -0,0 +1,37 @@
+// Copyright 2015 Google Inc. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// +build amd64 arm64 ppc64 ppc64le
+// +build go1.8
+
+// Assembly code isn't subject to visibility restrictions, so we can jump
+// directly into package runtime.
+//
+// Technique copied from here:
+//     https://github.com/golang/go/blob/d8c6dac/src/os/signal/sig.s
+
+#include "textflag.h"
+
+#ifdef GOARCH_ppc64
+#define JMP BR
+#endif
+#ifdef GOARCH_ppc64le
+#define JMP BR
+#endif
+
+TEXT ·memclr(SB),NOSPLIT,$0-16
+	JMP runtime·memclrNoHeapPointers(SB)
+
+TEXT ·memmove(SB),NOSPLIT,$0-24
+	JMP runtime·memmove(SB)

internal/buffer/runtime_other.s

@@ -13,6 +13,7 @@
 // limitations under the License.
 
 // +build amd64 arm64 ppc64 ppc64le
+// +build !go1.8
 
 // Assembly code isn't subject to visibility restrictions, so we can jump
 // directly into package runtime.