csu: test: explicitly add libm as build parameter

CSU tests build fails with '/usr/lib/libgcc_s.so: undefined reference to
fma' when built with LLVM 14 for powerpcspe, so '-lm' is being added
explicitly.

It may be linked to https://reviews.llvm.org/D77558

Reviewed by: imp (earlier version)
MFC after: 2 days
Sponsored by: Instituto de Pesquisas Eldorado (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D35691