x86-64: reduce symbol table size

x86-64: reduce symbol table size

With all of Xen's symbols sitting within a 2Gb range on x86-64, they
can be referred to by the kallsyms-like offset table using 4- instead
of 8-byte slots.

The marker table can use 4-byte slots in all cases, just like the
table entry counts can (though that's only a minor improvement).

If ia64's PERCPU_ADDR got moved down to (KERNEL_START + 2Gb -
PERCPU_PAGE_SIZE), it could also utilize the more compact form.

Signed-off-by: Jan Beulich <jbeulich@novell.com>