Currently, if a sigpanic call is injected into C code, it's possible
for preparePanic to leave the stack in a state where traceback can't
unwind correctly past the sigpanic.

Specifically, shouldPushPanic sniffs the stack to decide where to put
the PC from the signal context. In the cgo case, it will find that
!findfunc(pc).valid() because pc is in C code, and then it will check
if the top of the stack looks like a Go PC. However, this stack slot
is just in a C frame, so it could be uninitialized and contain
anything, including what looks like a valid Go PC. For example, in
https://build.golang.org/log/c601a18e2af24794e6c0899e05dddbb08caefc17,
it sees 1c02c23a <runtime.newproc1+682>. When this condition is met,
it skips putting the signal PC on the stack at all. As a result, when
we later unwind from the sigpanic, we'll "successfully" but
incorrectly unwind to whatever PC was in this uninitialized slot and
go who knows where from there.

Fix this by making shouldPushPanic assume that the signal PC is always
usable if we're running C code, so we always make it appear like
sigpanic's caller.

This lets us be pickier again about unexpected return PCs in
gentraceback.

Updates #23640.

Change-Id: I1e8ade24b031bd905d48e92d5e60c982e8edf160
Reviewed-on: https://go-review.googlesource.com/91137
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>