Add a define for index of IA32_XSS MSR, which is, per SDM rev. 50, an

analog of XCR0 for ring 0 FPU state, used by XSAVES and XRSTORS.

Sponsored by:	The FreeBSD Foundation
MFC after:	1 week

sys/x86/include/specialreg.h

@@ -490,6 +490,8 @@
 #define	MSR_APIC_DCR_TIMER	0x83e
 #define	MSR_APIC_SELF_IPI	0x83f
 
+#define	MSR_IA32_XSS		0xda0
+
 /*
  * Constants related to MSR's.
  */