x86: add a Kconfig symbol for when relocations are needed

We only need to build relocations when we are building a 32-bit
relocatable kernel.  Rather than unnecessarily complicating the
Makefiles, make an explicit Kbuild symbol for this.

[ Impact: permits future cleanup ]

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Cc: Sam Ravnborg <sam@ravnborg.org>