wireguard: selftests: use maximum cpu features and allow rng seeding

By forcing the maximum CPU that QEMU has available, we expose additional
capabilities, such as the RNDR instruction, which increases test
coverage. This then allows the CI to skip the fake seeding step in some
cases. Also enable STRICT_KERNEL_RWX to catch issues related to early
jump labels when the RNG is initialized at boot.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>