Fixes #3456.

This proposal is a reformulation of CL 5987063. This CL resets
the default GOARM value to 6 and allows the use of the VFPv3
optimisation if GOARM=7. Binaries built with this CL in place
will abort if GOARM=7 was used and the target host does not
support VFPv3.

R=minux.ma, rsc, ajstarks
CC=golang-dev
https://golang.org/cl/6501099