Use custom util instead of std::to_string.

std::to_string is not avcaliable on mingw.

--
MOS_MIGRATED_REVID=98923935
diff --git a/src/main/cpp/blaze_util_linux.cc b/src/main/cpp/blaze_util_linux.cc
index e18a853..8f0218a 100644
--- a/src/main/cpp/blaze_util_linux.cc
+++ b/src/main/cpp/blaze_util_linux.cc
@@ -132,7 +132,7 @@
 string GetProcessCWD(int pid) {
   char server_cwd[PATH_MAX] = {};
   if (readlink(
-          ("/proc/" + std::to_string(pid) + "/cwd").c_str(),
+          ("/proc/" + ToString(pid) + "/cwd").c_str(),
           server_cwd, sizeof(server_cwd)) < 0) {
     return "";
   }