configure: disable vfpv3 on armv6

VFPv3 is ARMv7-only, enabling it for ARMv6 is therefore always wrong.
Mea culpa, I do believe I'm the original author of that change.

Fixes: https://github.com/iojs/io.js/issues/283
PR-URL: https://github.com/iojs/io.js/pull/559


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