x86: fix lifting kernel CPU requirements and always enable PAE

commit 89878f60 x86: lift kernel minimum CPU requirement to Pentium MMX
caused kconfig havoc. Fix this and make sure PAE is enabled even on legacy
CPUs as the minimum required CPU has been Pentium MMX for a while now and
hence PAE is supported even on the x86_legacy target.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>