RISC-V: Fix feenableexcept return value

The man page says "The feenableexcept(), fedisableexcept(), and
fegetexcept() functions return a bitmap of the exceptions that were
unmasked prior to the call.", so we should return zero not -1.

Reviewed By:	mhorne
MFC after:	3 days
Differential Revision: https://reviews.freebsd.org/D29386

lib/msun/riscv/fenv.h

@@ -231,7 +231,7 @@ feenableexcept(int __mask __unused)
 
 	/* No exception traps. */
 
-	return (-1);
+	return (0);
 }
 
 static inline int