[Issue 23770] New: ImportC: error on gnu atomic builtins

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23770

          Issue ID: 23770
           Summary: ImportC: error on gnu atomic builtins
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Keywords: ImportC
          Severity: minor
          Priority: P4
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: dkorpel at live.nl

I'm getting an error compiling code that assumes gnu atomic builtins are
present, such as `__atomic_load_n`. Reduced:

```
#ifdef __TINYC__
#elif defined(_WIN32)
#else
int atomic_load(const volatile int *p)
{
#ifdef __ATOMIC_RELAXED
    return __atomic_load_n(p, __ATOMIC_RELAXED);
#endif
}
#endif

```

```
Error: undefined identifier `__atomic_load_n`
```

Other builtins:
https://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html

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