Fix tests on win64

These commits fix all tests and installation utilities on win64. I've confirmed that these are a green build on the bots.
The linux/osx builds download both 32/64 bit compilers to test cross compilation, and I'd like to enable cross compilation tests for windows soon as well.

In the meantime, the *cargo snapshot* is still only 32-bit for windows, in which case when building a 64-bit cargo we download the 32-bit *rust snapshot* to get out the `libgcc_s_dw2` dependency.