Silence unused parameter warnings in the RISC-V fenv.h

After increasing the lib/msun/tests WARNS to 6, this triggers a
compilation error for RISC-V.

Fixes: 87d65c747a43 ("lib/msun: Allow building tests with WARNS=6")
Reported by: Jenkins

(cherry picked from commit 15211f19509282d9c9a418d4e5b6ac75d9d1fc85)