- Stop defining custom architectures because VisualCpp toolchain doesn't
allow it. Reference to built-in architectures 'x86' and 'x86_64' since
they are supported by all toolchains.
- Remove 'local_arch' from platforms. For unsupported platform, we just
do not specify the target.
- Target no more than one platform at a time. This simplifies the build
script a lot.
- Remove the TARGET_ARCHS environment variable. Add system property
``arch`` to override ``osdetector.arch``.
- Add ``vc.disable`` to override the default choice of VisualCpp on
Windows.
- Add ``vc.`` prefix to the properties that are only used with VC++
Kun Zhang