i386: divorce CONFIG_X86_PAE from CONFIG_HIGHMEM64G

PAE is useful for more than supporting more than 4GB RAM.  It supports
expanded swapspace and NX executable protections.  Some users may want NX
or expanded swapspace support without the overhead or instability of
highmem.  For these reasons, the following patch divorces CONFIG_X86_PAE
from CONFIG_HIGHMEM64G.

Cc: Mark Lord <lkml@rtr.ca>
Signed-off-by: William Irwin <wli@holomorphy.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>