Make MLX build on x64 macOS (#901)

The arm64 macbook pros are heavy and I usually care my intel one for
mobile, it would be nice if I can play with MLX on it.

To build with x64, user must pass `MLX_ENABLE_X64_MAC` to cmake:
CMAKE_ARGS='-DMLX_ENABLE_X64_MAC=ON' python setup.py

python/tests/mlx_tests.py

@@ -11,8 +11,8 @@ import numpy as np
 
 class MLXTestCase(unittest.TestCase):
     @property
-    def is_linux(self):
-        return platform.system() == "Linux"
+    def is_apple_silicon(self):
+        return platform.machine() == "arm64" and platform.system() == "Darwin"
 
     def setUp(self):
         self.default = mx.default_device()

python/tests/test_fast_sdpa.py

@@ -62,7 +62,7 @@ class TestFastSDPA(mlx_tests.MLXTestCase):
         B = 1
         H = 32
         dtypes = [np.float32]
-        if not self.is_linux:
+        if self.is_apple_silicon:
             dtypes.append(np.half)
 
         for SEQUENCE_LENGTH in [1, 7, 9, 32, 63, 67, 129, 400, 2000]:

python/tests/test_ops.py

@@ -1772,8 +1772,8 @@ class TestOps(mlx_tests.MLXTestCase):
         )
 
     def test_tensordot(self):
-        # No fp16 matmuls on linux
-        if self.is_linux:
+        # No fp16 matmuls on common cpu backend
+        if not self.is_apple_silicon:
             dtypes = [mx.float32]
         else:
             dtypes = [mx.float16, mx.float32]