msun: update Clang bug reference in fma test

LLVM bugzilla bug 8100 became issue #8472 with the migration to GitHub.

https://github.com/llvm/llvm-project/issues/8472
(cherry picked from commit 92927b8bcf51dcbcf99d633c1b3cab3cab2373ac)
(cherry picked from commit 6cd4450c6c9a83a0fd9430df7ec02c4c3203ad25)