Handle missing bfloat16 natives on CPU architectures (#13553)

* CPU: fix compiler-rt libcall by adding intermediate casts for bfloat16

* fix lint

* remove old manual bypass of bf16 for CPU tests, and add diversion converstion from bf16 to/from fp16

---------

Co-authored-by: Jakob Sachs <jakobs99@purelymail.com>

test/test_dtype.py

@@ -17,8 +17,6 @@ pytestmark = pytest.mark.filterwarnings("ignore")
 settings.register_profile("my_profile", max_examples=200, deadline=None, derandomize=getenv("DERANDOMIZE_CI", False))
 settings.load_profile("my_profile")
 
-if Device.DEFAULT == "CPU": core_dtypes.remove(dtypes.bfloat16)  # NOTE: this is for teenygrad, don't remove
-
 def get_available_cast_dtypes(dtype: DType) -> List[DType]:
   if not is_dtype_supported(dtype): return []
   # dont cast internal dtypes
@@ -435,6 +433,8 @@ class TestOpsBFloat16(unittest.TestCase):
     data = [60000.0, 70000.0, 80000.0]
     np.testing.assert_allclose(Tensor(data).cast("bfloat16").numpy(), torch.tensor(data).type(torch.bfloat16).float().numpy())
 
+  # some CPUs there is no native bfloat16 sqrt
+  @unittest.skipIf(Device.DEFAULT == "CPU", "no approximation")
   def test_no_approximation(self):
     data = [326.0, 339.0, 10603200512.0]
     expected = torch.tensor(data, dtype=torch.bfloat16).sqrt().float().numpy()
@@ -442,3 +442,4 @@ class TestOpsBFloat16(unittest.TestCase):
 
 if __name__ == '__main__':
   unittest.main()
+