runtime: correct various Go -> C function calls

Some things get converted.
Other things (too complex or too many C deps) get onM calls.
Other things (too simple) get #pragma textflag NOSPLIT.

After this CL, the offending function list is basically:
        - panic.c
        - netpoll.goc
        - mem*.c
        - race stuff
        - readgstatus
        - entersyscall/exitsyscall

LGTM=r, iant
R=golang-codereviews, r, iant
CC=dvyukov, golang-codereviews, khr
https://golang.org/cl/140930043

src/pkg/runtime/signal_unix.go

@@ -0,0 +1,13 @@
+// Copyright 2012 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 dragonfly freebsd linux netbsd openbsd solaris
+
+package runtime
+
+func sigpipe()
+
+func os_sigpipe() {
+	onM(sigpipe)
+}