gen_syscalls.py: make 'noweak' a list instead of a random order set

Python's Sets are not deterministic. This causes the following lines to
be emitted in random order in generated/syscall_dispatch.c

extern u32_t z_mrsh_k_object_release(u32_t arg1, ...
extern u32_t z_mrsh_k_object_access_grant(u32_t arg1, ...
extern u32_t z_mrsh_k_object_alloc(u32_t arg1, ...

Change noweak to a basic list.

Reproducibility regression introduced by commit 6564974b

Signed-off-by: Marc Herbert <marc.herbert@intel.com>