Bump minimal supported Windows version to Windows Vista (0x0600).

[marceltaeumel]

Also fixes a minor issue in MSVC Makefile for 64x64, which tried 0x0801 for Windows 8 and 0x1001 for Windows 10. The latter would actually be 0x0A00. See https://docs.microsoft.com/de-de/cpp/porting/modifying-winver-and-win32-winnt

Also in the Makefile for 64x64, define both _WIN32 and _WIN64. See https://docs.microsoft.com/en-us/cpp/preprocessor/predefined-macros

Also in the Makefile for 64x64, define both WIN32 and WIN64 because those can be used to identify the Windows platform in application code such as in processors/IA32/bochs. I am not aware of any #ifdef WIN64 at the moment. Most code uses #ifdef _WIN64 to check for that.

Note that, having defined both _WIN32 and _WIN64, code for 32-bit and 64-bit must always begin with #ifdef _WIN64 and only then #elif _WIN32.

I think that the MSVC compiler defines _WIN32 and _WIN64 automatically. Yet, now it is documented in the Makefile.

[marceltaeumel]

That error on TravisCI is misleading. All builds pass.

[krono]

wanna merge? care to first update build.win64x64/common/Makefile.msvc.tools ?