x86/boot: Simplify pagetable manipulation loops

x86/boot: Simplify pagetable manipulation loops

For __page_tables_{start,end} and L3 bootmap initialisation, the logic is
unnecesserily complicated owing to its attempt to use the LOOP instruction,
which results in an off-by-8 memory address owing to LOOP's termination
condition.

Rewrite both loops for improved clarity and speed.

Misc notes:
 * TEST $IMM, MEM can't macrofuse.  The loop has 0x1200 iterations, so pull
   the $_PAGE_PRESENT constant out into a spare register to turn the TEST into
   its %REG, MEM form, which can macrofuse.
 * Avoid the use of %fs-relative references.  %esi-relative is the more common
   form in the code, and doesn't suffer an address generation overhead.
 * Avoid LOOP.  CMP/JB isn't microcoded and faster to execute in all cases.
 * For a 4 interation trivial loop, even compilers unroll these.  The
   generated code size is a fraction larger, but this is init and the asm is
   far easier to follow.
 * Reposition the l2=>l1 bootmap construction so the asm reads in pagetable
   level order.

No functional change.

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