[POWERPC] Use __attribute__ in asm-powerpc

Pretty much everyone uses "__attribute__" or "attribute", no one uses
"__attribute".  This tweaks the three places in asm-powerpc where this
comes up.  While only asm-powerpc/types.h is interesting (for
userspace), I did asm-powerpc/processor.h as well for consistency.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>