Fix check whether GCC supports __atomic_ builtins

Summary:
C11 atomics are never used because:

* The program used for checking whether GCC supports
__atomic_ builtins fails with the following error:

```
  error: size mismatch in argument 2 of `__atomic_load`
   int test(int *x) { int y; __atomic_load(&x, &y, __ATOMIC_SEQ_CST); return x; }
```

* There is a typo when checking if CONF_GCC_SUPPORTS__ATOMICS equals YES,
resulting in PRIM_CFLAGS and PRIM_EXTRA_LIBRARIES never being set.

Reviewers: bgamari

Reviewed By: bgamari

Subscribers: rwbarton, erikd, carter

Differential Revision: https://phabricator.haskell.org/D5154

0 files changed, 0 insertions, 0 deletions