Add couple memory barries to serialize tdq_cpu_idle and tdq_load accesses.

This change fixes transient performance drops in some of my benchmarks,
vanishing as soon as I am trying to collect any stats from the scheduler.
It looks like reordered access to those variables sometimes caused loss of
IPI_PREEMPT, that delayed thread execution until some later interrupt.

MFC after:	3 days

(cherry picked from commit 1cfaa7d62ba4ad88a87e3d3c7d9b1c7fc51059a0)
Always report that we support REPORT TARGET PORT GROUPS command.

Without clustering support we any way have only one group of permanently
active ports, but that gives us one more supported VMWare feature. ;)

Solaris' Comstar also reports it even when only one port is present.

(cherry picked from commit 2969b41f6571148fe9a43b685086e4435fdf4a42)