UPSTREAM: ARM: early traps initialisation

commit 04e91b73 upstream.

Provide a couple of helpers to copy the vectors and stubs, and also
to flush the copied vectors and stubs.

Bug: 215557547
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: Id60ef859e431a093597199294e4f57efdde84867