Windows: package system runtime dependencies

This will make sure that system files that rust binaries depend on in Windows get packaged into stage0 snapshots as well as into Windows installer.
Currently these include `libgcc_s_dw2-1.dll`, `libstdc++-6.dll` and `libpthread-2.dll`.   Note that the latter will need to be changed to `pthreadGC2.dll` once Windows build bots get upgraded to mingw 4.0

Closes #9252
Closes #5878
Closes #9218
Closes #5712