net/openmpi: Update 5.0.7 => 5.0.8

Changelog:
https://docs.open-mpi.org/en/v5.0.8/release-notes/changelog/v5.0.x.html

Replace PORTVERSION with DISTVERSION in MASTER_SITES.
Fix warnings from portclippy.
Add workaround for gfortran on riscv64.

PR: 287212 287211