Fix PowerPC backtraces.  Since kernel and user have completely separate address
spaces, rather than a split address, we actually can't check for being within
the kernel's address range.  Instead, do what other backtraces do, and use
trapexit()/asttrapexit() as the stack sentinel.

MFC after:	3 weeks
Add DEF_CLONE_CMD_ARG2

This will be used in the forthcoming vxlan import.

Reviewed by:	gnn
Phabric:	https://reviews.freebsd.org/D382
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
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.
Use gic_decode_fdt() rather than a local routine to parse fdt interrupt
properties.  Move fdt_pic_table and fdt_fixup_table into imx6_machdep.c,
which means imx6 doesn't need imx_common.c anymore.