Use int64_t rather than int64

For cross platform and cross compiler portability we use the
standard integer type for a 64-bit integer. MinGW on Windows doesn't
have the definition for `int64`.

src/sysinfo.cc

@@ -243,7 +243,7 @@ void InitializeSystemInfo() {
           SHGetValueA(HKEY_LOCAL_MACHINE,
                       "HARDWARE\\DESCRIPTION\\System\\CentralProcessor\\0",
                       "~MHz", nullptr, &data, &data_size)))
-    cpuinfo_cycles_per_second = (int64)data * (int64)(1000 * 1000);  // was mhz
+    cpuinfo_cycles_per_second = (int64_t)data * (int64_t)(1000 * 1000);  // was mhz
   else
     cpuinfo_cycles_per_second = EstimateCyclesPerSecond();
 // TODO: also figure out cpuinfo_num_cpus