add linuxarmv6l to poco formula

Follow-on to the linuxarmv7l tweaks, but built and tested on a Raspberry Pi instead of ODROID-U3.

The build time was borderline horrific on the Raspberry Pi clocking in somewhere well over 2 hours - I wonder if debug binaries are being stripped at the build phase, maybe only the release version should be built? Maybe their Makefile doesn't support only release?