Revert "csu: test: explicitly add libm as build parameter"

This reverts commit 4f5890a0fb086324a657f3cd7ba1abc57274e0db.

9ef1127008ce94cf626daed346a3c1ee03063617 is a proper fix for
the problem we tried to address.

Sponsored by: Instituto de Pesquisas Eldorado (eldorado.org.br)