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: NH. Peter Anvin <hpa@zytor.com>
Cc: Sam Ravnborg <sam@ravnborg.org>

arch/x86/Kconfig

@@ -1513,6 +1513,11 @@ config RELOCATABLE
 	  it has been loaded at and the compile time physical address
 	  (CONFIG_PHYSICAL_START) is ignored.
 
+# Relocation on x86-32 needs some additional build support
+config X86_NEED_RELOCS
+	def_bool y
+	depends on X86_32 && RELOCATABLE
+
 config PHYSICAL_ALIGN
 	hex
 	prompt "Alignment value to which kernel should be aligned" if X86_32