arm64: Add alternate CONFIG_DEBUG_SET_MODULE_RONX support

Commit 802ae1e1
(arm64: mm: Introduce PTE_WRITE) changed pte_wrprotect
and pte_mkwrite to use software read only bits instead
of the hardware bits explicitly. The hardware read only
bits are updated in set_pte_at. Unfortunately, the check
in set_pte_at only works for user page tables which breaks
support for the read only functions in modules. Move to an
alternative implementation which does not use pte_mkwrite/pte_wrprotect.

Change-Id: I4251a0929b1fe6f43f84b14f0a64fed30769700e
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>