Make the loader code compatible with MinGW (2)

[illwieckz]

Replay of:

• Make the loader code compatible with MinGW #12

Does nothing until:

• Prepare for the CMake plumbing to build the loader with MinGW (2) #18
• CMake plumbing to build the loader (2) #14

[slipher]

win32: include stdint for INT64_MAX should be removed because win32: silence some MSVC pragma and redefinitions on MinGW already gets that include. Other than that LGTM.

Commit message nit: the phrase "posix intrinsics" doesn't make sense.

[illwieckz]

Oopsie, I forgot to rename that:

Commit message nit: the phrase "posix intrinsics" doesn't make sense.

Not a big deal though.