x86-32: move _end to a dummy section

Impact: build fix with CONFIG_RELOCATABLE

Move _end into a dummy section, so that relocs.c will know it is a
relocatable symbol.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Cc: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>