We will move the CPU feature overrides into BSS in a subsequent patch,
and this requires that BSS is zeroed before the feature override
detection code runs. So let's map BSS read-write in the ID map, and zero
it via this mapping.

Since the kernel page tables are right next to it, and also zeroed via
the ID map, let's drop the separate clear_page_tables() function, and
just zero everything in one go.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20240214122845.2033971-51-ardb+git@google.comSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>