Update installation instructions for Windows.
* README.windows: Assume a 64-bit Windows. Explain both 32-bit and 64-bit
builds. Add instructions for the MS Visual C/C++ tool chain. Revamp instructions
for Cygwin.
* gettext-runtime/README.windows: Generic reference to top-level file.
* gettext-runtime/intl/README.windows: Likewise.
* gettext-runtime/libasprintf/README.windows: Likewise.
* gettext-tools/README.windows: Likewise.
