OpenWatcom: Move non-Windows settings to Modules/Compiler

Restructure OpenWatcom toolchain support files to simplify adding of new targets DOS, OS/2 and Linux including cross-compilation.

In Modules/Compiler/OpenWatcom.cmake file are all common setup specific for toolchain which doesn't depend on target platform or architecture.

Modules/Platform/Windows-OpenWatcom.cmake now hold only Windows specific toolchain setup.

Topic-rename: openwatcom-platform-independence