ppc: Check for compilers that don't support relocation

Certain ppc compilers are known not to generate the .fixup section
properly.  The .fixup section is necessary to create a relocatable
U-Boot image.  A basic check for the existence of the .fixup section
should hopefully catch the majority of broken compilers which don't
support relocation.

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>