selftests/nolibc: introduce QEMU_ARCH_USER

While ppc64le shares the same executable with regular ppc64 the user
variant needs has a dedicated executable.
Introduce a new QEMU_ARCH_USER Makefile variable to accommodate that.

Fixes: 17362f3d ("selftests/nolibc: use qemu-system-ppc64 for ppc64le")
Link: https://lore.kernel.org/r/20770915-nolibc-run-user-v1-1-3caec61726dc@weissschuh.net


Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>