[POWERPC] Always use -mno-string

The string load/store instructions are unimplemented on some processors
and slow (microcoded) on some others.  It's simplest to just not use
them at all.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>