x86/boot: Support __ro_after_init

x86/boot: Support __ro_after_init

For security hardening reasons, it advantageous to make setup-once data
immutable after boot.  Borrow __ro_after_init from Linux.

On x86, place .data.ro_after_init at the start of .rodata, excluding it from
the early permission restrictions.  Re-apply RO restrictions to the whole of
.rodata in init_done(), attempting to reform the superpage if possible.

For architectures which don't implement __ro_after_init explicitly, variables
merges into .data.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>