The exception handler modifies the stack and continuation context so
it looks like the faulting code calls sigpanic() directly. The call was
not set up correctly on ARM, because it did not handle the link register
correctly. This change handles the link register correctly for ARM.

Updates #28854

Change-Id: I7ccf838adfc05cd968a5edd7d19ebba6a2478360
Reviewed-on: https://go-review.googlesource.com/c/150957
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>