Fix Linux to Windows cross compilation of py_binary, java_binary and sh_binary using MinGW

[jesseschalken]

trafficstars

This branch

• Fixes various compilation and runtime errors when building the C++ Windows launcher using a MinGW toolchain on Linux
• Fixes Windows target platform detection to use the target platform instead of the host platform
• Removes the execution platform configuration from the $launcher attributes so that the launcher is compiled for the target platform
• Adds target platform constraints to the autogenerated Windows toolchains so that they can be selected with target platform constraints as a replacement for --compiler=... when using --incompatible_enable_cc_toolchain_resolution

[sgowroji]

Hello @jesseschalken, Could you please resolve the code conflicts. I have shared it to team, for further review.

[jesseschalken]

@sgowroji Thanks. I've merged master but I am yet to fix the Windows test failures (I'm still trying to get them to pass locally on master). Hoping someone beats me to it.

[comius]

Tests might be failing because --platforms=windows isn't passed (or some autodetection is failing).