Add Apple GPUs (#49)

This fixes testing on local macOS machines (CI doesn't have this problem
because they don't attach GPUs)

Author: keith

mojo/extensions.bzl

@@ -204,6 +204,10 @@ _gpu_toolchains_tag = tag_class(
                 "mi300x": "amdgpu:gfx942",
                 "mi325": "amdgpu:gfx942",
                 "rtx5090": "nvidia:120a",
+                "m1": "metal:10",
+                "m2": "metal:20",
+                "m3": "metal:30",
+                "m4": "metal:40",
             },
             doc = "The GPUs supported by this toolchain, mapping to Mojo's target accelerators.",
         ),
@@ -227,6 +231,10 @@ _gpu_toolchains_tag = tag_class(
                 "MI325": "mi325",
                 "Navi": "radeon",
                 "AMD Radeon Graphics": "radeon",
+                "Apple M1": "m1",
+                "Apple M2": "m2",
+                "Apple M3": "m3",
+                "Apple M4": "m4",
             },
             doc = "The output from nvidia-smi or rocm-smi to the corresponding GPU name in SUPPORTED_GPUS.",
         ),