[POWERPC] Add files build to .gitignore

Mostly taken from corresponding Makefile's make-clean rule.

Tested by (cross)compiling for $ARCH PPC and POWERPC and checking
output of git-status.

Signed-off-by: Rutger Nijlunsing <git-commit@tux.tmfweb.nl>
Signed-off-by: Paul Mackerras <paulus@samba.org>