Package system runtime dependencies into snapshots
Package system runtime dependencies into Windows distribution.
auto merge of #9662 : vadimcn/rust/package-runtime-deps, r=brson

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