UPSTREAM: arm: kgdb: Handle read-only text / modules

Handle the case where someone has set the text segment of the kernel
as read-only by using the newly introduced "patch" mechanism.

Signed-off-by: Doug Anderson <dianders@chromium.org>
[kees: switched structure size check to BUILD_BUG_ON (sboyd)]
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Nicolas Pitre <nico@linaro.org>

(cherry picked from commit 23a4e4050ba9c98ab67db0980a9fb20e5096d9ea)
Signed-off-by: Kees Cook <keescook@google.com>
Change-Id: I39fafcb198d37779650e9ddb0461cf28406beeed