arm64: Add support for STACKLEAK gcc plugin

arm64: Add support for STACKLEAK gcc plugin

This adds support for the STACKLEAK gcc plugin to arm64 by implementing
stackleak_check_alloca(), based heavily on the x86 version, and adding the
two helpers used by the stackleak common code: current_top_of_stack() and
on_thread_stack(). The stack erasure calls are made at syscall returns.
Additionally, this disables the plugin in hypervisor and EFI stub code,
which are out of scope for the protection.

Acked-by: Alexander Popov <[email protected]>
Reviewed-by: Mark Rutland <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Signed-off-by: Laura Abbott <[email protected]>
Signed-off-by: Will Deacon <[email protected]>