[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/sigqueue.go

@@ -45,7 +45,7 @@ const (
 
 // Called from sighandler to send a signal back out of the signal handling thread.
 // Reports whether the signal was sent. If not, the caller typically crashes the program.
-func sigsend(s int32) bool {
+func sigsend(s uint32) bool {
 	bit := uint32(1) << uint(s&31)
 	if !sig.inuse || s < 0 || int(s) >= 32*len(sig.wanted) || sig.wanted[s/32]&bit == 0 {
 		return false
@@ -157,9 +157,6 @@ func badsignal(sig uintptr) {
 	cgocallback(unsafe.Pointer(funcPC(sigsend)), noescape(unsafe.Pointer(&sig)), unsafe.Sizeof(sig))
 }
 
-func sigenable_m()
-func sigdisable_m()
-
 func sigenable_go(s uint32) {
 	g := getg()
 	g.m.scalararg[0] = uintptr(s)