fixup! Import the source code of mimalloc v2.1.2

To support mimalloc-enabled builds also on Linux and on macOS, we need
to ship the respective backends, too.

This helped me diagnose "mimalloc vs reftable" problems much quicker
because Git's birthplace *is* Linux, and its support for debugging on
that platform is still highly unmatched by its support for any other
platform.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

compat/mimalloc/prim/osx/prim.c

@@ -0,0 +1,9 @@
+/* ----------------------------------------------------------------------------
+Copyright (c) 2018-2023, Microsoft Research, Daan Leijen
+This is free software; you can redistribute it and/or modify it under the
+terms of the MIT license. A copy of the license can be found in the file
+"LICENSE" at the root of this distribution.
+-----------------------------------------------------------------------------*/
+
+// We use the unix/prim.c with the mmap API on macOSX
+#include "../unix/prim.c"