[PowerPC] Implement VDSO timebase access on powerpc*

Implement the remaining pieces needed to allow userland timestamp reading.

Rewritten based on an intial essay into the problem by Justin Hibbits.
(Copyright changed to my own on his request.)

Tested on ppc64 (POWER9 Talos II), powerpcspe (e500v2 RB800), and
powerpc (g4 PowerBook).

Reviewed by: jhibbits (in irc)
Sponsored by: Tag1 Consulting, Inc.
Differential Revision: https://reviews.freebsd.org/D26347