x86/generic: use HIGHMEM64G instead of HIGHMEM4G to fix PAE and Xen

x86/generic: use HIGHMEM64G instead of HIGHMEM4G to fix PAE and Xen

This is a backport of 641a65fd062987a456216cc4fa91ff2910528261 in master.

This change re-enables PAE for the 32-bit x86 subtarget, which is
interesting in its own right but also necessary for Xen support.

Commit af1d1ebd ("x86: enable 4G high memory support for generic (32bit)
subtarget") inadvertently disabled both PAE and Xen support.

Fixes: FS#908

Cc: Daniel Golle <daniel@makrotopia.org>
Cc: Jo-Philipp Wich <jo@mein.io>
Signed-off-by: Baptiste Jonglez <git@bitsofnetworks.org>