Handle platform dependent start method (#179)

In Python multiprocessing, start method name is platform dependent [1].
For macOS, it is "spawn" for reasons listed in [2], vs. "fork" for
Linux.

[1] https://docs.python.org/3/library/multiprocessing.html#multiprocessing.get_start_method
[2] python/cpython#77906

python/test/unit/runtime/test_subproc.py

@@ -49,7 +49,8 @@ def kernel_dot(Z):
 
 def test_compile_in_forked_subproc(fresh_triton_cache) -> None:
     config = AttrsDescriptor.from_hints({0: 16})
-    assert multiprocessing.get_start_method() == 'fork'
+    # This can be either fork or spawn, depending on the platform.
+    assert multiprocessing.get_start_method() in ["fork", "spawn"]
     proc = multiprocessing.Process(target=compile_fn_dot, args=(config, ))
     proc.start()
     proc.join()
@@ -92,7 +93,9 @@ def test_compile_in_forked_subproc_with_forced_gc(fresh_triton_cache) -> None:
 
     # stage 2.p
     shutil.rmtree(fresh_triton_cache)
-    assert multiprocessing.get_start_method() == 'fork'
+    # This can be either fork or spawn, depending on the platform.
+    assert multiprocessing.get_start_method() in ["fork", "spawn"]
+
     proc = multiprocessing.Process(target=compile_empty_kernel_with_gc, args=(config, ))
 
     # stage 3.c