runtime: don't inject a sigpanic if not on user G stack

runtime: don't inject a sigpanic if not on user G stack

If a panicking signal (e.g. SIGSEGV) happens on a g0 stack, we're
either in the runtime or running C code. Either way we cannot
recover and sigpanic will immediately throw. Further, injecting a
sigpanic could make the C stack unwinder and the debugger fail to
unwind the stack. So don't inject a sigpanic.

If we have cgo traceback and symbolizer attached, if it panics in
a C function ("CF" for the example below), previously it shows
something like

	fatal error: unexpected signal during runtime execution
	[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x45f1ef]

	runtime stack:
	runtime.throw({0x485460?, 0x0?})
		.../runtime/panic.go:1076 +0x5c fp=0x7ffd77f60f58 sp=0x7ffd77f60f28 pc=0x42e39c
	runtime.sigpanic()
		.../runtime/signal_unix.go:821 +0x3e9 fp=0x7ffd77f60fb8 sp=0x7ffd77f60f58 pc=0x442229

	goroutine 1 [syscall]:
	CF
		/tmp/pp/c.c:6 pc=0x45f1ef
	runtime.asmcgocall
		.../runtime/asm_amd64.s:869 pc=0x458007
	runtime.cgocall(0x45f1d0, 0xc000053f70)
		.../runtime/cgocall.go:158 +0x51 fp=0xc000053f48 sp=0xc000053f10 pc=0x404551
	main._Cfunc_CF()
		_cgo_gotypes.go:39 +0x3f fp=0xc000053f70 sp=0xc000053f48 pc=0x45f0bf

Now it shows

	SIGSEGV: segmentation violation
	PC=0x45f1ef m=0 sigcode=1
	signal arrived during cgo execution

	goroutine 1 [syscall]:
	CF
		/tmp/pp/c.c:6 pc=0x45f1ef
	runtime.asmcgocall
		.../runtime/asm_amd64.s:869 pc=0x458007
	runtime.cgocall(0x45f1d0, 0xc00004ef70)
		.../runtime/cgocall.go:158 +0x51 fp=0xc00004ef48 sp=0xc00004ef10 pc=0x404551
	main._Cfunc_CF()
		_cgo_gotypes.go:39 +0x3f fp=0xc00004ef70 sp=0xc00004ef48 pc=0x45f0bf

I think the new one is reasonable.

For #57698.

Change-Id: I4f7af91761374e9b569dce4c7587499d4799137e
Reviewed-on: https://go-review.googlesource.com/c/go/+/462437
Reviewed-by: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Cherry Mui <cherryyz@google.com>