Apple started calling their operating system macOS, rather than OSX a while ago,
and as such, we should follow suit. This introduces `::hostos::MACOS`, and makes
`::OSX` an alias of it.
All internal users were updated to refer to `::hostos::MACOS`, but the `::OSX`
alias is being kept indefinitely.
Signed-off-by: Gergely Nagy <algernon@keyboard.io>