mach: Use PAGE_SIZE

The PAGE_SIZE from the Mach headers statically defines the machine's
page size. There's no need to query it dynamically; furthermore, the
implementation of the vm_statistics () RPC unconditionally fills in

pagesize = PAGE_SIZE;

Not doing the extra RPC shaves off 2 RPCs from the start-up of every
process!

Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>
Message-Id: <20230218203717.373211-7-bugaevc@gmail.com>