runtime: fix signal handling on Solaris

This fixes the problems with signal handling that were inadvertently
introduced in https://go-review.googlesource.com/21006.

Fixes #14899

Change-Id: Ia746914dcb3146a52413d32c57b089af763f0810
Reviewed-on: https://go-review.googlesource.com/21145Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/signal_solaris_amd64.go

@@ -11,10 +11,6 @@ type sigctxt struct {
 	ctxt unsafe.Pointer
 }
 
-func makesigctxt(info *siginfo, ctxt unsafe.Pointer) *sigctxt {
-	return &sigctxt{info, ctxt}
-}
-
 func (c *sigctxt) regs() *mcontext {
 	return (*mcontext)(unsafe.Pointer(&(*ucontext)(c.ctxt).uc_mcontext))
 }

src/runtime/sys_solaris_amd64.s

@@ -173,10 +173,9 @@ TEXT runtime·sigtramp(SB),NOSPLIT,$0
 	MOVQ	g(BX), R10
 	CMPQ	R10, $0
 	JNE	allgood
-	MOVQ	SI, 0(SP)
-	MOVQ	DX, 8(SP)
-	CALL	runtime·makesigctxt(SB)
-	MOVQ	16(SP), AX
+	MOVQ	SI, 80(SP)
+	MOVQ	DX, 88(SP)
+	LEAQ	80(SP), AX
 	MOVQ	DI, 0(SP)
 	MOVQ	AX, 8(SP)
 	MOVQ	$runtime·badsignal(SB), AX