[Windows] Correctly choose 32-bit version of mimalloc-redirect{,32}.dll in CMake.

[mkurdej]

This fixes the use of mimalloc on 32-bit Windows when building with CMake.
Without it, one would get link errors like:

      init.obj error LNK2019: unresolved external symbol __imp__mi_allocator_init referenced in function "void __cdecl mi_process_load(void)" (?mi_process_load@@YAXXZ)
10:58:29
      init.obj error LNK2019: unresolved external symbol __imp__mi_allocator_done referenced in function "void __cdecl mi_process_done(void)" (?mi_process_done@@YAXXZ)
10:58:29
      ..\..\..\..\thirdparty\mimalloc\bin\mimalloc-redirect.lib warning LNK4272: library machine type 'x64' conflicts with target machine type 'x86'

because it would try to link bin\mimalloc-redirect.lib (64-bit) to a 32-bit mimalloc.dll.

Also, without this patch, bin\mimalloc-redirect32.{lib,dll} are unused in CMake, they are used though in .vcxproj files.

[daanx]

Great, thanks so much