devel/llvm16: backport upstream powerpc FP fix

The upstream commit fde5924dcc69 has been merged to llvm in the FreeBSD
main branch so merge it here as well.

Issue: https://github.com/llvm/llvm-project/issues/63542
(cherry picked from commit 2a9d785d67c6d66268d46f4d3ff0f255dc01e9ea)