build: Warn when using snapshot on Armv6

Building io.js on armv6 with snapshot enabled does not work due to a bug in v8, so it now is disabled by default when building on armv6. However I added a --force-snapshot option so the user can enable snapshot for debuging purposes.

I found this problem while trying to build io.js on my Raspberry Pi. With snapshot disabled v8 and all of the code compiles , but still when testing io.js I get an 'illegal instruction' error. I´ll open an issue for that.