As noted in bug 1518166, we need to be able to compile with a MinGW-win64 targetting toolchain in Taskcluster. (win32 one already exists)

My preferred approach is to modify the scripts that build the win32 toolchain so it can also build a win64 one.

Thunderbird requires a MinGW toolchain for x64 for bug 1518166 (OTR support
in chat). The required libraries cannot be successfully compiled with the
Clang+MinGW cross compilers nor Clang-CL on Windows.

These changes should not have any impact on existing code that builds the
win32 toolchain.

If an x64 toolchain is required, build-gcc-mingw32.sh is run with the argument
x86_64-w64-mingw32. If no argument is given, the default is i686-w64-mingw32,
the same as the current behavior.

Thunderbird plans to include libotr and its dependencies in future builds in
order to support Off-the-record messaging.
When building these libraries for Windows, GCC/MinGW is currently needed.
This Docker image includes the Debian MinGW compiler and will be used for
building libotr, libgcrypt, and libgpg-error only for Windows. The build
artifacts will then be included in the Thunderbird build.
The associated Taskcluster configuration will go in the comm- repository only so
it won't affect Firefox.

https://hg.mozilla.org/mozilla-central/rev/e5ce8377af87

Pushed by thunderbird@calypsoblue.org:
https://hg.mozilla.org/comm-central/rev/55ccb8667c98
MinGW Docker image for Thunderbird libotr. r=darktrojan

https://hg.mozilla.org/comm-central/rev/55ccb8667c98