Fix atomic fetch operations

[chjj]

The atomic_fetch ops are supposed to return the old value of the object. chibicc is currently returning the new values.

stdatomic.h:

#define atomic_fetch_add(obj, val) (*(obj) += (val))
#define atomic_fetch_sub(obj, val) (*(obj) -= (val))
#define atomic_fetch_or(obj, val) (*(obj) |= (val))
#define atomic_fetch_xor(obj, val) (*(obj) ^= (val))
#define atomic_fetch_and(obj, val) (*(obj) &= (val))

I was originally thinking about how to fix this in a simple way. Three of them (add/sub/xor) are fixable with macros. Something like:

#define atomic_fetch_add(obj, val) ({ \
  typeof(val) _val = (val);           \
  ((*(obj) += _val) - _val);          \
})

#define atomic_fetch_sub(obj, val) ({ \
  typeof(val) _val = (val);           \
  ((*(obj) -= _val) + _val);          \
})

#define atomic_fetch_xor(obj, val) ({ \
  typeof(val) _val = (val);           \
  ((*(obj) ^= _val) ^ _val);          \
})

However, atomic_fetch_and and atomic_fetch_or are not fixable via macro as they are not reversible.

This PR adds a single new builtin (__builtin_atomic_fetch_op) which reuses the existing atomic operation code.