gcc: install AMD intrinsics header ammintrin.h

This file was included in the changes from r251212 and originated
in the pre-GPLv3 gcc43 branch.

MFC after:	2 weeks
Do not report current link status if driver is not running.
Reporting link status in driver has a side-effect that makes mii(4)
check current link status.  mii(4) will call link status change
callback when it sees link state change.  Normally this wouldn't
have problems. However, ASF/IPMI firmware can actively access PHY
regardless of driver's running state such that reporting link
status for not-running interface can generate meaningless link
UP/DOWN messages.

This change also makes dhclient think driver got a valid link
regardless of link establishment so it will bypass dhclient's
initial link status check. I think that wouldn't be issue
though.

Tested by:	Daniel Braniss <danny@cs.huji.ac.il>
Correct setting TX random backoff register. This register is
implemented as a 10 bits linear feedback shift register so only
lower 10 bits are valid.
Because this register is used to initialize random backoff interval
register only when resolved duplex is half-duplex, it wouldn't have
caused issues in these days.

Submitted by:	Masanobu SAITOH <msaitoh@NetBSD.org>