Generalise the va to pa code and use it when starting secondary cores

Reviewed by:	ian@, rpaulo@
Differential Revision: https://reviews.freebsd.org/D736


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271232 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f