powerpc/pmap64: Make moea64 statistics optional

Summary:
It turns out statistics accounting is very expensive in the pmap driver,
and doesn't seem necessary in the common case. Make this optional
behind a MOEA64_STATS #define, which one can set if they really need
statistics.

This saves ~7-8% on buildworld time on a POWER9.

Found by bdragon.

Reviewed by: luporl
Differential Revision: https://reviews.freebsd.org/D20903