Add configuration for armv6 with vfp.

The RPi is armv6 but has vfp.  My understanding is that Spidermonkey is much faster with vfp.  This armv6-vfp arch target is only used for the upcoming RPi device (which hopefully is the last armv6 device in the world).  I don't know of any way this config target could affect any other of the platform_build files.

Goal is to merge this into master.  I don't know of any reason create a separate branch for this config, since it's purely additive.

r? @michaelwu