comms/libusbsio: fix build on architectures where uname -p != uname -m

The source uses uname -m to build a subdirectory, but we used uname -p
to describe it. A fix was already attempted, but only incidentally
correct on ppc. Use the right variable to fix the build on arm64, too.

Approved by: portmgr (build fix blanket)

(cherry picked from commit 2effd5a3db89793cb70bedd71740c3c339e08ac4)