Handle EMMS x86 instruction in libmach.

This is notably useful to be able to use Acid on Go programs.

Reference: /n/sources/patch/libmach-8db-emms
Date: Thu Aug 28 09:49:25 CES 2014
Signed-off-by: djc@9grid.fr

--- /sys/src/libmach/8db.c	Thu Aug 28 09:49:13 2014
+++ /sys/src/libmach/8db.c	Thu Aug 28 12:08:08 2014
@@ -535,6 +535,7 @@
 [0x74]	RM,0,		"PCMPEQB %m,%M",
 [0x75]	RM,0,		"PCMPEQW %m,%M",
 [0x76]	RM,0,		"PCMPEQL %m,%M",
+[0x77]	0,0,		"EMMS",
 [0x7E]	RM,0,		"MOV%S %M,%e",
 [0x7F]	RM,0,		"MOVQ %M,%m",
 [0xAE]	RMOP,0,		optab0FAE,