kgdb on amd64

[Patrick Welche <prlw1%cam.ac.uk@localhost>]

The reason kgdb wasn't working all this time on amd64 is that GETC()
returns -1 immediately whether or not a character is available =>
all of kgdb's checksums fail due to the extra "-1" characters.

A quick revert of

RCS file: /cvsroot/src/sys/dev/ic/com.c,v
----------------------------
revision 1.322
date: 2013-12-22 18:20:46 +0000;  author: matt;  state: Exp;  lines: +7 -5;  commitid: T4pi7xZzU6djP9ix;
Now that cngetc supports -1 as a return value from the cn_getc routine,
let cngetc loop until a character is available so it can call the critpoll
hooks.
----------------------------

gets something vaguely sensible:

Packet qSupported (supported-packets) is NOT supported
Sending packet: $Hg0#df...Ack
Packet received: 
Sending packet: $qTStatus#49...Ack
Packet received: 
Packet qTStatus (trace-status) is NOT supported
Sending packet: $?#3f...Ack
Packet received: S05
Sending packet: $Hc-1#09...Ack
Packet received: 
Sending packet: $qC#b4...Ack
Packet received: 
Sending packet: $qAttached#8f...Ack
Packet received: 

=> counts as "analyzed", so a fix is in sight...

Cheers,

Patrick