selftests/nolibc: rename riscv to riscv64

riscv32 support is about the be added. To keep the naming clear and
consistent with other architectures rename riscv to riscv64, as that is
what it actually represents.

Link: https://lore.kernel.org/r/20241221-nolibc-rv32-v1-5-d9ef6dab7c63@weissschuh.net


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