Add an ARMv7 toolchain to cloudabi-toolchain.

CloudABI recently gained separate packages for ARMv7 based systems.
These systems can freely use ARMv7-specific features like NEON, whereas
the ARMv6 packages will need to stick to just VFP.

This change extends the cloudabi-toolchain package to also set up a
toolchain for ARMv7 properly.

Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D10938