runtime: properly restore registers in Solaris runtime·sigtramp

We restored registers correctly in the usual case where the thread
is a Go-managed thread and called runtime·sighandler, but we
failed to do so when runtime·sigtramp was called on a cgo-created
thread. In that case, runtime·sigtramp called runtime·badsignal,
a Go function, and did not restore registers after it returned

LGTM=rsc, dave
R=rsc, dave
CC=golang-codereviews, minux.ma
https://golang.org/cl/105280050

src/pkg/runtime/sys_solaris_amd64.s

@@ -164,7 +164,7 @@ TEXT runtime·sigtramp(SB),NOSPLIT,$0
 	MOVQ	DI, 0(SP)
 	MOVQ	$runtime·badsignal(SB), AX
 	CALL	AX
-	RET
+	JMP	exit
 
 allgood:
 	// save g
@@ -259,6 +259,7 @@ allgood:
 	MOVQ	80(SP), R10
 	MOVQ	R10, g(BX)
 
+exit:
 	// restore registers
 	MOVQ	32(SP), BX
 	MOVQ	40(SP), BP