runtime, runtime/cgo: revert CL 18814; don't drop signal stack in new thread on dragonfly

This change reverts CL 18814 which is a workaroud for older DragonFly
BSD kernels, and fixes #13945 and #13947 in a more general way the
same as other platforms except NetBSD.

This is a followup to CL 29491.

Updates #16329.

Change-Id: I771670bc672c827f2b3dbc7fd7417c49897cb991
Reviewed-on: https://go-review.googlesource.com/29971
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/cgo/gcc_dragonfly_amd64.c

@@ -56,7 +56,6 @@ static void*
 threadentry(void *v)
 {
 	ThreadStart ts;
-	stack_t ss;
 
 	ts = *(ThreadStart*)v;
 	free(v);
@@ -66,17 +65,6 @@ threadentry(void *v)
 	 */
 	setg_gcc((void*)ts.g);
 
-	// On DragonFly, a new thread inherits the signal stack of the
-	// creating thread. That confuses minit, so we remove that
-	// signal stack here before calling the regular mstart. It's
-	// a bit baroque to remove a signal stack here only to add one
-	// in minit, but it's a simple change that keeps DragonFly
-	// working like other OS's. At this point all signals are
-	// blocked, so there is no race.
-	memset(&ss, 0, sizeof ss);
-	ss.ss_flags = SS_DISABLE;
-	sigaltstack(&ss, nil);
-
 	crosscall_amd64(ts.fn);
 	return nil;
 }

src/runtime/sys_dragonfly_amd64.s

@@ -51,18 +51,6 @@ TEXT runtime·lwp_start(SB),NOSPLIT,$0
 	MOVQ	R13, g_m(DI)
 	MOVQ	DI, g(CX)
 
-	// On DragonFly, a new thread inherits the signal stack of the
-	// creating thread. That confuses minit, so we remove that
-	// signal stack here before calling the regular mstart. It's
-	// a bit baroque to remove a signal stack here only to add one
-	// in minit, but it's a simple change that keeps DragonFly
-	// working like other OS's. At this point all signals are
-	// blocked, so there is no race.
-	SUBQ	$8, SP
-	MOVQ	$0, 0(SP)
-	CALL	runtime·signalstack(SB)
-	ADDQ	$8, SP
-
 	CALL	runtime·stackcheck(SB)
 	CALL	runtime·mstart(SB)