mimalloc: silence -Wconstant-conversion warnings

```
../../../system/lib/mimalloc/src/os.c:36:3: error: implicit conversion from 'unsigned long long' to 'size_t' (aka 'unsigned long') changes value from 4294967296 to 0 [-Werror,-Wconstant-conversion]
   32 | static mi_os_mem_config_t mi_os_mem_config = {
      |                                              ~
   33 |   4096,     // page size
   34 |   0,        // large page size (usually 2MiB)
   35 |   4096,     // allocation granularity
   36 |   MI_DEFAULT_PHYSICAL_MEMORY,
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~
../../../system/lib/mimalloc/src/os.c:26:40: note: expanded from macro 'MI_DEFAULT_PHYSICAL_MEMORY'
   26 | #define MI_DEFAULT_PHYSICAL_MEMORY    4*MI_GiB
      |                                       ~^~~~~~~
1 error generated.
```

Author: kleisauke

tools/system_libs.py

@@ -1796,6 +1796,7 @@ class libmimalloc(MTLibrary):
     '-Wno-unused-but-set-variable',
     '-Wno-unused-variable',
     '-Wno-deprecated-pragma',
+    '-Wno-constant-conversion',
     # build emmalloc as only a system allocator, without exporting itself onto
     # malloc/free in the global scope
     '-DEMMALLOC_NO_STD_EXPORTS',