runtime: fix 386 signal handler bug

Cannot assume that g == m->curg at time of signal.
Must save actual g and restore.

Fixes flaky crashes with messages like

throw: malloc mlookup
throw: malloc/free - deadlock
throw: unwindstack on self
throw: free mlookup

(and probably others) when running cgo.

R=iant
CC=golang-dev
https://golang.org/cl/1648043

src/pkg/runtime/darwin/386/sys.s

@@ -74,8 +74,13 @@ TEXT sigaction(SB),7,$0
 //	16(FP)	siginfo
 //	20(FP)	context
 TEXT sigtramp(SB),7,$40
-	// g = m->gsignal
 	get_tls(CX)
+
+	// save g
+	MOVL	g(CX), BP
+	MOVL	BP, 20(SP)
+
+	// g = m->gsignal
 	MOVL	m(CX), BP
 	MOVL	m_gsignal(BP), BP
 	MOVL	BP, g(CX)
@@ -91,10 +96,9 @@ TEXT sigtramp(SB),7,$40
 	MOVL	CX, 8(SP)
 	CALL	DI
 
-	// g = m->curg
+	// restore g
 	get_tls(CX)
-	MOVL	m(CX), BP
-	MOVL	m_curg(BP), BP
+	MOVL	20(SP), BP
 	MOVL	BP, g(CX)
 
 	MOVL	context+16(FP), CX

src/pkg/runtime/linux/386/sys.s

@@ -56,22 +56,39 @@ TEXT rt_sigaction(SB),7,$0
 	INT	$0x80
 	RET
 
-TEXT sigtramp(SB),7,$0
+TEXT sigtramp(SB),7,$40
 	get_tls(CX)
-	MOVL	m(CX), BP
-	MOVL	m_gsignal(BP), AX
-	MOVL	AX, g(CX)
-	JMP	sighandler(SB)
+
+	// save g
+	MOVL	g(CX), BX
+	MOVL	BX, 20(SP)
+
+	// g = m->gsignal
+	MOVL	m(CX), BX
+	MOVL	m_gsignal(BX), BX
+	MOVL	BX, g(CX)
+
+	// copy arguments for call to sighandler
+	MOVL	sig+0(FP), BX
+	MOVL	BX, 0(SP)
+	MOVL	info+4(FP), BX
+	MOVL	BX, 4(SP)
+	MOVL	context+8(FP), BX
+	MOVL	BX, 8(SP)
+
+	CALL	sighandler(SB)
+
+	// restore g
+	get_tls(CX)
+	MOVL	20(SP), BX
+	MOVL	BX, g(CX)
+
+	RET
 
 TEXT sigignore(SB),7,$0
 	RET
 
 TEXT sigreturn(SB),7,$0
-	// g = m->curg
-	get_tls(CX)
-	MOVL	m(CX), BP
-	MOVL	m_curg(BP), BP
-	MOVL	BP, g(CX)
 	MOVL	$173, AX	// rt_sigreturn
 	INT $0x80
 	INT $3	// not reached
@@ -259,4 +276,3 @@ TEXT setldt(SB),7,$32
 	MOVW	AX, GS
 
 	RET
-