plat/x86: Enable NX bit when initializing paging

plat/x86: Enable NX bit when initializing paging

There are a few reasons to enable the NX bit in the paging init routine:
* Setting the bit before checking for actual support is risky.
  Currently, the check happens in the paging initialization routine.
* The bit is not used, when the Unikraft paging support is not enabled.
* Every boot entry point (32-bit/64-bit) would have a duplicated enable.

Signed-off-by: Marco Schlumpp <marco@unikraft.io>
Reviewed-by: Florin Postolache <florin.postolache80@gmail.com>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #752