lib: arm/arm64: Use WRITE_ONCE to update the translation tables

Use WRITE_ONCE to prevent store tearing when updating an entry in the
translation tables. Without WRITE_ONCE, the compiler, even though it is
unlikely, can emit several stores when changing the table, and we might
end up with bogus TLB entries.

It's worth noting that the existing code is mostly fine without any
changes because the translation tables are updated in one of the
following situations:

- When the tables are being created with the MMU off, which means no TLB
  caching is being performed.

- When new page table entries are added as a result of vmalloc'ing a
  stack for a secondary CPU, which doesn't happen very often.

- When clearing the PTE_USER bit for the cache test, and store tearing
  has no effect on the table walker because there are no intermediate
  values between bit values 0 and 1. We still use WRITE_ONCE in this case
  for consistency.

However, the functions are global and there is nothing preventing someone
from writing a test that uses them in a different scenario. Let's make
sure that when that happens, there will be no breakage once in a blue
moon.

Reported-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>