Add the ppoll() system call.
Export kern_poll() needed by an upcoming Linuxulator change.

Differential Revision:	https://reviews.freebsd.org/D1133
Reviewed by:	kib, wblock
MFC after:	1 month


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274462 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
Regen for r274462.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274463 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f