gdb, gdbarch: Introduce gdbarch method to get the shadow stack pointer.

This patch is required by the following commit
"gdb: Enable displaced stepping with shadow stack on amd64 linux."

Reviewed-By: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
Approved-By: Luis Machado <luis.machado@arm.com>
Approved-By: Andrew Burgess <aburgess@redhat.com>