crc: Use absl::nullopt when returning absl::optional

Otherwise we can observe a build failure when absl::optional != std::optional.

PiperOrigin-RevId: 716275922
Change-Id: I4918a8901530f0daafeec07e319fd79123358bc1

Author: majnemer

absl/crc/internal/cpu_detect.cc

@@ -296,7 +296,7 @@ static absl::optional<T> ReadSysctlByName(const char* name) {
   size_t val_size = sizeof(T);
   int ret = sysctlbyname(name, &val, &val_size, nullptr, 0);
   if (ret == -1) {
-    return std::nullopt;
+    return absl::nullopt;
   }
   return val;
 }