loader: Add TSLOG to interpreter built-ins

Wrap each call to a built-in command with TSENTER/TSEXIT to make
it easier to see where time is going in the loader.

MFC after: 1 week
Sponsored by: Amazon
Differential Revision: https://reviews.freebsd.org/D46630

(cherry picked from commit 58ec2d0d4a3c3bcf741d9694dc7696c7b4e753b9)