[libc][Obvious] Add a default clause to RoundingModeUtils.h to suppress gcc

warning.

libc/utils/testutils/RoundingModeUtils.cpp

@@ -27,6 +27,8 @@ int get_fe_rounding(RoundingMode mode) {
   case RoundingMode::Nearest:
     return FE_TONEAREST;
     break;
+  default:
+    __builtin_unreachable();
   }
 }