Make use of `quotRemInt#` primop in `quotRemInteger`

Otoh, `divModInt#` is not a proper primop (it's implemented as wrapper
around `quotRemInt#` in `GHC.Base`), so we can't do the same for
`divModInteger` yet.

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>
Refactor `stgReallocForGMP` to use `memcpy`

GCC is able to generate better code when using `memcpy` instead of
manually copying bytes in a loop. Otoh, `stgAllocForGMP` is typically
called for enlarging initial single-limb structures (see also #8647 for
more information) and so this minor optimization won't be very visible
in measurements.

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>