selftests/nolibc: skip tests for unimplemented syscalls

The riscv32 architecture is missing many of the older syscalls.
Instead of providing wrappers for everything at once, introducing a lot
of complexity, skip the tests for those syscalls for now.

Link: https://lore.kernel.org/r/20241221-nolibc-rv32-v1-4-d9ef6dab7c63@weissschuh.net


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