Add amd64 code.  Various formatting changes.
 [rsc] --rw-rw-r-- M 191333 glenda sys 72374 Sep 22 12:10 386/lib/libthread.a
 [rsc] --rw-rw-r-- M 191333 rsc sys 579 Sep 22 12:10 sys/src/libthread/amd64.c
 [rsc] --rw-rw-r-- M 191333 glenda sys 1012 Sep 22 12:10 sys/src/libthread/mkfile
	/n/sourcesdump/2005/0922/plan9/sys/src/libthread/mkfile:32,37 - 
	/n/sourcesdump/2005/0923/plan9/sys/src/libthread/mkfile:32,38
	  CFILES=\
	  	386.c\
	  	alpha.c\
	+ 	amd64.c\
	  	arm.c\
	  	channel.c\
	  	chanprint.c\
	/n/sourcesdump/2005/0922/plan9/sys/src/libthread/mkfile:61,72 - 
	/n/sourcesdump/2005/0923/plan9/sys/src/libthread/mkfile:62,74
	  	test.c\
	  	tprimes.c\
	  	xincarm.c\
	- 	xincpower.c
	  
	  SFILES=\
	  	xinc386.s\
	  	xincalpha.s\
	- 	xincmips.s
	+ 	xincamd64.s\
	+ 	xincmips.s\
	+ 	xincpower.s\
	  
	  HFILES=\
	  	/sys/include/thread.h\
 [rsc] --rw-rw-r-- M 191333 rsc sys 292 Sep 22 12:10 sys/src/libthread/xincamd64.s
 [rsc] --rw-rw-r-- M 191333 glenda sys 211 Sep 22 12:10 sys/src/libthread/xincport.h
	/n/sourcesdump/2005/0922/plan9/sys/src/libthread/xincport.h:5,11 - 
	/n/sourcesdump/2005/0923/plan9/sys/src/libthread/xincport.h:5,12
	  static Lock l;
	  
	  void
	- _xinc(long *p) {
	+ _xinc(long *p)
	+ {
	  
	  	lock(&l);
	  	(*p)++;
	/n/sourcesdump/2005/0922/plan9/sys/src/libthread/xincport.h:13,19 - 
	/n/sourcesdump/2005/0923/plan9/sys/src/libthread/xincport.h:14,21
	  }
	  
	  long
	- _xdec(long *p) {
	+ _xdec(long *p)
	+ {
	  	long r;
	  
	  	lock(&l);