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