sysutils/opensbi: update to v0.8

This brings some restructuring to the installation paths. Firmware
files are still stored under /usr/local/share, but with the new prefix
allowing for multiple ABIs to be installed (e.g. opensbi/lp64/qemu/virt).
Libraries have been moved to /usr/local/lib64.

A new "generic" platform has been added, which will produce firmware
binaries capable of running on several platforms. The qemu/virt and
spike platforms have been removed as a result.

Additionally, the fw_payload.[bin|elf] files will no longer be
installed. These are compiled with the test payload and therefore don't
offer much use as part of this port.

Reviewed by: lbartoletti (previous version)
Approved by: lbartoletti
Differential Revision: https://reviews.freebsd.org/D24660