powerpc/boot: allow wrapper to work on non-english system

if the language is not english objdump output is not parsed correctly
and format is "". Later, "ld -m $format" fails.

This patch adds "LANG=C" to force english output for objdump.

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>