Print CPU informtion later in boot.

Match other architectures and print CPU information during
cpu_startup(). In particular, this prints the information after the
message buffer is initialized which allows it to be retrieved after
boot via dmesg(8).

While here, add some extern declarations to <machine/md_var.h> in
place of duplicated declarations in various source files.

Reviewed by: brooks
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D24936