add case for arm architecture to work on RP2

[codefoster]

@jasongin, I added the line like you suggested and just tested it on my RP2. Worked great. Thank you. Here's a PR.

[codefoster]

@jasongin, I'm not certain this is working as expected. When I use it on my RP2, I get the 6l (that's a lowercase L) version and not 7l as I would expect. I don't have time to look into the reason right now, but the process.config.variables.arm_version might be incorrect.

[jasongin]

See these related node issues:

• process.config.variables.arm_version incorrect in armv7
• api to get arm architecture of the binary as in the download page

So, it's a bug in node, and I think not one that I can reasonably work around. But it should have little or no impact in practice, since the arm v6 binary should run fine.