gh-90815: Add mimalloc memory allocator (#109914)

* Add mimalloc v2.12

Modified src/alloc.c to remove include of alloc-override.c and not
compile new handler.

Did not include the following files:

 - include/mimalloc-new-delete.h
 - include/mimalloc-override.h
 - src/alloc-override-osx.c
 - src/alloc-override.c
 - src/static.c
 - src/region.c

mimalloc is thread safe and shares a single heap across all runtimes,
therefore finalization and getting global allocated blocks across all
runtimes is different.

* mimalloc: minimal changes for use in Python:

 - remove debug spam for freeing large allocations
 - use same bytes (0xDD) for freed allocations in CPython and mimalloc
   This is important for the test_capi debug memory tests

* Don't export mimalloc symbol in libpython.
* Enable mimalloc as Python allocator option.
* Add mimalloc MIT license.
* Log mimalloc in Lib/test/pythoninfo.py.
* Document new mimalloc support.
* Use macro defs for exports as done in:
  https://github.com/python/cpython/pull/31164/

Co-authored-by: Sam Gross <colesbury@gmail.com>
Co-authored-by: Christian Heimes <christian@python.org>
Co-authored-by: Victor Stinner <vstinner@python.org>

Lib/test/test_cmd_line.py

@@ -803,6 +803,7 @@ class CmdLineTest(unittest.TestCase):
     def test_pythonmalloc(self):
         # Test the PYTHONMALLOC environment variable
         pymalloc = support.with_pymalloc()
+        mimalloc = support.with_mimalloc()
         if pymalloc:
             default_name = 'pymalloc_debug' if support.Py_DEBUG else 'pymalloc'
             default_name_debug = 'pymalloc_debug'
@@ -821,6 +822,11 @@ class CmdLineTest(unittest.TestCase):
                 ('pymalloc', 'pymalloc'),
                 ('pymalloc_debug', 'pymalloc_debug'),
             ))
+        if mimalloc:
+            tests.extend((
+                ('mimalloc', 'mimalloc'),
+                ('mimalloc_debug', 'mimalloc_debug'),
+            ))
 
         for env_var, name in tests:
             with self.subTest(env_var=env_var, name=name):