[dev.cc] runtime: convert signal handlers from C to Go

This code overused macros and could not be
converted automatically. Instead a new sigctxt
type had to be defined for each os/arch combination,
with a common (implicit) interface used by the
arch-specific signal handler code.

[This CL is part of the removal of C code from package runtime.
See golang.org/s/dev.cc for an overview.]

LGTM=r
R=r
CC=austin, dvyukov, golang-codereviews, iant, khr
https://golang.org/cl/168500044

src/runtime/signal_unix.go

@@ -6,8 +6,6 @@
 
 package runtime
 
-func sigpipe()
-
 func os_sigpipe() {
 	onM(sigpipe)
 }