Fixed possible precision loss in microtime

This is related to the fix for bug #64370. MSVC natively supports __int64 type,
so calculating with 32 bit ints is neither necessary nor reliable. Therefore
an older piece of code is reused.
Merge branch 'PHP-5.3' into PHP-5.4

* PHP-5.3:
  Fixed possible precision loss in microtime
Merge branch 'PHP-5.4' into PHP-5.5

* PHP-5.4:
  Fixed possible precision loss in microtime