selftests/nolibc: use qemu-system-ppc64 for ppc64le

qemu-system-ppc64 can handle both big and little endian kernels.

While some setups, like Debian, provide a symlink to execute
qemu-system-ppc64 as qemu-system-ppc64le, others, like ArchLinux, do not.

So always use qemu-system-ppc64 directly.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Link: https://lore.kernel.org/lkml/20231008-nolibc-qemu-ppc64-v1-1-29e2326e0420@weissschuh.net/


Acked-by: Willy Tarreau <w@1wt.eu>