powerpc: Get rid of CROSS32{AS,LD,OBJCOPY}

CROSS32AS and CROSS32LD are never used (instead, CROSS32CC is used
with proper command line options).

CROSS32OBJCOPY isn't used anymore either, since the "wrapper" stuff
was added.

Remove these unused variables.

Signed-off-by: Segher Boessenkool <segher@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>