fixup! [libc][math] Optimize nearest integer functions using builtins when available

Fix GCC warning "variable ‘result’ set but not used".

Author: overmighty

libc/test/src/math/performance_testing/nearest_integer_funcs_perf.cpp

@@ -42,7 +42,7 @@ template <typename T> class NearestIntegerPerf {
                                 StorageType ending_bit, StorageType step,
                                 size_t rounds, std::ofstream &log) {
     auto runner = [=](Func func) {
-      volatile T result;
+      [[maybe_unused]] volatile T result;
       for (size_t i = 0; i < rounds; i++) {
         for (StorageType bits = starting_bit; bits <= ending_bit;
              bits += step) {