[POWERPC] Fix compiler/assembler flags for Ebony platform boot files

The recent addition of assembler flags for 44x.c and ebony.c in the
bootwrapper to make them compile on certain toolchains was not correct
and could break other platforms.  This patch switches to using a
compiler flag instead, which implies the appropriate assembler flag,
and also stops the compiler itself generating instructions which are
invalid for the platform in question.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Acked-by: Segher Boessenkool <segher@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>