atomicFetchAdd and atomicFetchSub are not supported

[rymrg]

atomicFetchAdd and atomicFetchSub are not supported.
Adding support in ldc-developers/druntime#193

import std;
import core.atomic;
void main()
{
    long x = 1;
    writeln(x.atomicFetchAdd(1));
    writeln(x.atomicFetchSub(1));
    writeln(x);
}

Output:

/dlang/ldc-1.23.0/bin/../import/core/atomic.d(191): Error: undefined identifier atomicFetchAdd in module core.internal.atomic
onlineapp.d(6): Error: template instance core.atomic.atomicFetchAdd!(cast(MemoryOrder)5, long) error instantiating
/dlang/ldc-1.23.0/bin/../import/core/atomic.d(220): Error: undefined identifier atomicFetchSub in module core.internal.atomic
onlineapp.d(7): Error: template instance core.atomic.atomicFetchSub!(cast(MemoryOrder)5, long) error instantiating

[rymrg]

Merged.