Use HW_PHYSMEM in get_total_system_memory_impl
An upcoming PR will change
get_total_system_memory_impl to use
HW_PHYSMEM instead of
HW_USERMEM in the case where
HW_PHYSMEM is defined but some other methods are not available for getting the total physical memory (one example: FreeBSD).
The code actually checks that
HW_PHYSMEM is defined, and comments reference
HW_USERMEM is simply used instead.
For OpenBSD, NetBSD and OSX a 64-bit variant of
HW_PHYSMEM is used when available.
The same PR will also contain a commit to update/fix a couple of comments in this same file. It will fix a typo in a comment and update another comment which suggests that
HW_PHYSMEM64 is something only on OpenBSD when NetBSD actually defines it as well.