astro/oskar: fix build on powerpc* with clang

There are some Clang-on-PPC-specific issues.

Clang doesn't have ppu_intrinsics.h. mulhdu and mulhwu are required.

PR: 242016
Approved by: yuri (maintainer), linimon (mentor)
Differential Revision: https://reviews.freebsd.org/D22424