runtime, cmd/gc, cmd/ld: ignore method wrappers in recover

Bug #1:

Issue 5406 identified an interesting case:
        defer iface.M()
may end up calling a wrapper that copies an indirect receiver
from the iface value and then calls the real M method. That's
two calls down, not just one, and so recover() == nil always
in the real M method, even during a panic.

[For the purposes of this entire discussion, a wrapper's
implementation is a function containing an ordinary call, not
the optimized tail call form that is somtimes possible. The
tail call does not create a second frame, so it is already
handled correctly.]

Fix this bug by introducing g->panicwrap, which counts the
number of bytes on current stack segment that are due to
wrapper calls that should not count against the recover
check. All wrapper functions must now adjust g->panicwrap up
on entry and back down on exit. This adds slightly to their
expense; on the x86 it is a single instruction at entry and
exit; on the ARM it is three. However, the alternative is to
make a call to recover depend on being able to walk the stack,
which I very much want to avoid. We have enough problems
walking the stack for garbage collection and profiling.
Also, if performance is critical in a specific case, it is already
faster to use a pointer receiver and avoid this kind of wrapper
entirely.

Bug #2:

The old code, which did not consider the possibility of two
calls, already contained a check to see if the call had split
its stack and so the panic-created segment was one behind the
current segment. In the wrapper case, both of the two calls
might split their stacks, so the panic-created segment can be
two behind the current segment.

Fix this by propagating the Stktop.panic flag forward during
stack splits instead of looking backward during recover.

Fixes #5406.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/13367052

src/pkg/runtime/runtime.h

diff --git a/src/pkg/runtime/runtime.h b/src/pkg/runtime/runtime.h
index 151804f..9974fa3 100644
--- a/src/pkg/runtime/runtime.h
+++ b/src/pkg/runtime/runtime.h
@@ -250,6 +250,8 @@
 	// stackguard0 can be set to StackPreempt as opposed to stackguard
 	uintptr	stackguard0;	// cannot move - also known to linker, libmach, runtime/cgo
 	uintptr	stackbase;	// cannot move - also known to libmach, runtime/cgo
+	uint32	panicwrap;	// cannot move - also known to linker
+	uint32	selgen;		// valid sudog pointer
 	Defer*	defer;
 	Panic*	panic;
 	Gobuf	sched;
@@ -264,7 +266,6 @@
 	void*	param;		// passed parameter on wakeup
 	int16	status;
 	int64	goid;
-	uint32	selgen;		// valid sudog pointer
 	int8*	waitreason;	// if status==Gwaiting
 	G*	schedlink;
 	bool	ispanic;
@@ -403,6 +404,7 @@
 	uintptr	stackbase;
 	Gobuf	gobuf;
 	uint32	argsize;
+	uint32	panicwrap;
 
 	uint8*	argp;	// pointer to arguments in old frame
 	uintptr	free;	// if free>0, call stackfree using free as size