runtime: make arm signal handler call runtime.badsignal

In preparation for CL 9249043 (already submitted).

Fixes #5553.

R=golang-dev, iant, capnm9, dave
CC=golang-dev
https://golang.org/cl/9251043

src/pkg/runtime/sys_freebsd_arm.s

@@ -154,12 +154,17 @@ TEXT runtime·sigtramp(SB),7,$24
 	// this might be called in external code context,
 	// where g and m are not set.
 	// first save R0, because _cgo_load_gm will clobber it
-	// TODO(adonovan): call runtime·badsignal if m=0, like other platforms?
 	MOVW	R0, 4(R13) // signum
 	MOVW	_cgo_load_gm(SB), R0
 	CMP 	$0, R0
 	BL.NE	(R0)
 
+	CMP $0, m
+	BNE 3(PC)
+	// signal number is already prepared in 4(R13)
+	BL runtime·badsignal(SB)
+	RET
+
 	// save g
 	MOVW R10, R4
 	MOVW R10, 20(R13)

src/pkg/runtime/sys_linux_arm.s

@@ -286,12 +286,17 @@ TEXT runtime·sigtramp(SB),7,$24
 	// this might be called in external code context,
 	// where g and m are not set.
 	// first save R0, because _cgo_load_gm will clobber it
-	// TODO(adonovan): call runtime·badsignal if m=0, like other platforms?
 	MOVW	R0, 4(R13)
 	MOVW	_cgo_load_gm(SB), R0
 	CMP 	$0, R0
 	BL.NE	(R0)
 
+	CMP 	$0, m
+	BNE 	3(PC)
+	// signal number is already prepared in 4(R13)
+	BL  	runtime·badsignal(SB)
+	RET
+
 	// save g
 	MOVW	g, R3
 	MOVW	g, 20(R13)

src/pkg/runtime/sys_netbsd_arm.s

@@ -201,12 +201,17 @@ TEXT runtime·sigtramp(SB),7,$24
 	// this might be called in external code context,
 	// where g and m are not set.
 	// first save R0, because _cgo_load_gm will clobber it
-	// TODO(adonovan): call runtime·badsignal if m=0, like other platforms?
 	MOVW	R0, 4(R13) // signum
 	MOVW	_cgo_load_gm(SB), R0
 	CMP 	$0, R0
 	BL.NE	(R0)
 
+	CMP $0, m
+	BNE 3(PC)
+	// signal number is already prepared in 4(R13)
+	BL runtime·badsignal(SB)
+	RET
+
 	// save g
 	MOVW R10, R4
 	MOVW R10, 20(R13)