We are setting CONFIG_GEN_PRIV_STACKS when AArch64 actually uses a
statically allocated privileged stack.
This error was not captured by the tests because we only verify whether
a read/write to a privileged stack is failing, but it can fail for a lot
of reasons including when the pointer to the privileged stack is not
initialized at all, like in this case.
With this patch we deselect CONFIG_GEN_PRIV_STACKS and we fix the
mem_protect/userspace test to correctly probe the privileged stack.
Signed-off-by: Carlo Caione <ccaione@baylibre.com>