Handle SWAPGS.
 [jmk] --rw-rw-r-- M 174505 glenda sys 50731 Nov 24 01:13 sys/src/libmach/8db.c
	/n/sourcesdump/2005/1124/plan9/sys/src/libmach/8db.c:299,307 - 
	/n/sourcesdump/2005/1125/plan9/sys/src/libmach/8db.c:299,312
	  [0x03]	0,0,		"MOVL	%e,IDTR",
	  [0x04]	0,0,		"MOVW	MSW,%e",	/* word */
	  [0x06]	0,0,		"MOVW	%e,MSW",	/* word */
	- [0x07]	0,0,		"INVLPG	%e",		/* TO DO: distinguish SWAPGS? */
	+ [0x07]	0,0,		"INVLPG	%e",		/* or SWAPGS */
	  };
	  
	+ static Optable optab0F01F8[1]=
	+ {
	+ [0x00]	0,0,		"SWAPGS",
	+ };
	+ 
	  /* 0F71 */
	  /* 0F72 */
	  /* 0F73 */
	/n/sourcesdump/2005/1124/plan9/sys/src/libmach/8db.c:1571,1578 - 
	/n/sourcesdump/2005/1125/plan9/sys/src/libmach/8db.c:1576,1585
	  				return 0;
	  			if (modrm(map, ip, c) < 0)
	  				return 0;
	- 			c = ip->reg;
	  			obase = (Optable*)op->proto;
	+ 			if(ip->amd64 && obase == optab0F01 && c == 0xF8)
	+ 				return optab0F01F8;
	+ 			c = ip->reg;
	  			goto newop;
	  		case FRMOP:	/* FP R/M field with op code (/digit) */
	  			if (igetc(map, ip, &c) < 0)