Emulate "POP r/m".

This is needed to boot OpenBSD/i386 MP kernel in bhyve.

Reported by:	grehan
MFC after:	1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273108 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f