MFC r234691

MFC r234691

 Implement the D "cpu" variable, which returns curcpu.  I have chosen not
 to follow the example of OpenSolaris and its descendants, which implemented
 cpu as an inline that took a value out of curthread.  At certain points in
 the FreeBSD scheduler curthread->td_oncpu will no longer be valid (in
 particukar, just before the thread gets descheduled) so instead I have
 implemented this as its own built-in variable.

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git-svn-id: svn://svn.freebsd.org/base/stable/8@236161 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f