Prepare for the CMake plumbing to build the loader with MinGW (2)

[illwieckz]

Replay of:

• Prepare for the CMake plumbing to build the loader with MinGW #7

Does nothing until:

• Make the loader code compatible with MinGW (2) #17

[illwieckz]

Hmm, in fact the CMake plumbing isn't in that PR, but in the general CMake PR. Let's rename it.

[illwieckz]

I may mention this in the commit message, but it works because all that is needed is two macros from macamd64.inc, and ksamd64.inc included macamd64.inc so it was overkill to include ksamd64.inc but it worked.

When stubbing it we better make it as short as possible.

On MSVC, the standard macamd64.inc will be used, on MinGW the stub will be used by adding the directory containing it to the list of include directories.

[illwieckz]

Also if I remember correctly the standard .inc files use some formatting variants (especially in comment formatting or other useless things like that) that are incompatible with JWasm. The stub is kept as minimal as possible and use a formatting meant to please JWasm. By luck the NaCl .asm file is already compatible with both MASM and JWasm.