runtime: fix bp restoration in panic recovery for arm64
Previously, the frame pointer wouldn't be restored at all, which could
cause panics during frame pointer unwinding. As of CL 516157, the frame
pointer is restored, but it's restored incorrectly on arm64: on arm64,
the frame pointer points one word below SP, but here it's one below
panic.fp which is the stack pointer of the caller's frame (nothing to do
with the architectural bp).
For #61766.
Change-Id: I86504b85a4d741df5939b51c914d9e7c8d6edaad
Reviewed-on: https://go-review.googlesource.com/c/go/+/523697
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>