build: add asan option to multi-arch/mode build

Make it possible to build node against Address Sanitizer.  Enable with:

    $ make -f Makefile.build asan=clang++ CC=clang CC_host=clang

PR-URL: https://github.com/node-forward/node/pull/24


Reviewed-By: Fedor Indutny <fedor@indutny.com>