Replace mempcpy by mempcpy to fix #3589 : mempcpy is not available on Mac OS

`mempcpy` is not available on Mac OS and the only difference between `memcpy` and `mempcpy` is the return value. As long as the return value is not used we can use memcpy (which is available in Mac OS)

THis PR will also fix htps://github.com/mcuadros/homebrew-hhvm/issues/97