Bootstrap fails when Windows has `uname` command

[ajpaverd]

bootstrap.py assumes Windows platforms do not have the uname command in the PATH. Although almost always true, this doesn't hold if the gnuwin32 tools are installed (e.g. for running LLVM tests), in which case the output is:

> uname -s
windows32
> uname -m
i686-pc

Bootstrap then gives the following error:

> python x.py test src/tools/tidy
Updating only changed submodules
Submodules updated in 0.56 seconds
unknown OS type: windows32
Build completed unsuccessfully in 0:00:00

@rustbot modify labels: +A-rustbuild