x86, boot: Simplify setting of the PAE bit

A single 'movl' is shorter than the 'xorl'-'orl' pair.
No change in behaviour.

Signed-off-by: Alexander Potashev <aspotashev@gmail.com>
LKML-Reference: <1256341043-4928-1-git-send-email-aspotashev@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>