Use VirtualAlloc() on WIN32

[aitap]

Hello again,

Here's the pull request I've been meaning to submit back in 2020. This makes use of VirtualAlloc on WIN32 to allocate memory dynamically instead of relying on INTERNAL_MEMORY_SPACE. After ./configure --host=x86_64-w64-mingw32 I can rename and manually run the dmalloc_fc_t and dmalloc_t executables with the arguments specified for the light and heavy Makefile targets; the tests pass.