MFC r271604, r271616:
Add couple memory barriers to order 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 r271403: Add PCI ID for Promise TX8660 8-port 3Gbps HBA.

This device reports RAID subclass, but appears to be AHCI compatible.

Submitted by:	Yuri Perejilin <yuri@rivera.ru>