src: prefix ARCH and PLATFORM with NODE_

The PLATFORM preprocessor symbol is defined in node.gyp, and on Windows
it's set to "win". This conflicts with a built-in preprocessor symbol
with a different value ("win32"), which makes the linker(!) complain.
Resolve this by renaming these symbols to NODE_ARCH and NODE_PLATFORM.

PR-URL: https://github.com/iojs/io.js/pull/261


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>