Update Windows toolchain to a more recent version (GCC 8.2.0 + MinGW v6.0.0)

The current toolchain used in cerbero for Windows is quite old, both the GCC compiler version and the MinGW one. It's time for an update to catch up with new windows headers and projects that requires more recent GCC versions. The proposed versions are GCC-8.2.0 and MinGW 6.0.0 with the same configuration as before: posix threads and SJLJ exceptions.

At some point we should move away from SJLJ exceptions and use DWARF2 for x86 and SEH for x86_64 so that we are compatible with MSYS2 builds, which is what most people are using nowadays. Maybe this is a good moment for the switch too, for example QT did this move already: https://wiki.qt.io/MinGW-64-bit.

EDIT: The new toolchains are finally configured to used SEH for x86_64 and SJLJ for x86.

• Create Window multilib toolchain for Linux x86_64
• Create Window multilib toolchain for Windows x86_64
• Upload Window multilib toolchain for Linux x86_64 freedesktop
• Upload Window multilib toolchain for Windows x86_64 freedesktop
• Test a full cross-compilation build from Linux for Windows x86_64
• Test a full cross-compilation build from Linux for Windows x86
• Test a full cross-compilation build from Windows for Windows x86_64
• Test a full cross-compilation build from Windows for Windows x86
• Write documentation