Replace atomic.h and __atomic usage with stdatomic.h

[letmaik]

The following files include the internal musl header atomic.h:

• src/enclave/enclave_event_channel.c
  • __atomic_fetch_add, __atomic_add_fetch, __atomic_load_n, __atomic_compare_exchange_n
  • Note that no a_* functions are used, so the header include is redundant.
• src/include/enclave/lthread.h
  • a_crash, a_spin
• src/include/enclave/ticketlock.h
  • a_fetch_add_uint, a_spin, a_barrier, a_cas_64
• src/sched/futex.c
  • a_barrier, a_fetch_add

We should eventually switch to the C11 stdatomic.h header to avoid the dependency on internal musl headers and compiler-specific __atomic builtins.