Adjust MemSet macro to use size_t rather than long

Adjust MemSet macro to use size_t rather than long

Likewise for MemSetAligned.

"long" wasn't the most suitable type for these macros as with MSVC in
64-bit builds, sizeof(long) == 4, which is narrower than the processor's
word size, therefore these macros had to perform twice as many loops as
they otherwise might.

Author: David Rowley <dgrowleyml@gmail.com>
Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://postgr.es/m/CAApHDvoGFjSA3aNyVQ3ivbyc4ST=CC5L-_VjEUQ92HbE2Cxovg@mail.gmail.com