gn build: Unbreak get.py and gn.py on Windows

`os.uname()` doesn't exist on Windows, so use `platform.machine()` which
returns `os.uname()[4]` on non-Win and (on 64-bit systems) "AMD64" on Windows.
Also use `sys.platform` instead of `platform` to check for Windows-ness for the
file extension in gn.py (get.py got this right).

Differential Revision: https://reviews.llvm.org/D59115

llvm-svn: 355693