mips: hide regnum definitions behind _KERNEL/_WANT_MIPS_REGNUM

machine/regnum.h ends up being included by sys/procfs.h and sys/ptrace.h via
machine/reg.h. Many of the regnum definitions are too short and too generic
to be exposing to any userland application including one of these two
headers. Moreover, these actively cause build failures in googletest
(template <typename T1 ...> expanding to template <typename 9 ...>).

Hide the definitions behind _KERNEL or _WANT_MIPS_REGNUM, and patch all of
the userland consumers to define as needed.

Discussed with: imp, jhb
Reviewed by: imp, jhb
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D21330