x86: Initial support for WRMSRNS

WRMSR Non-Serialising is an optimisation intended for cases where an MSR needs
updating, but architectural serialising properties are not needed.

In is anticipated that this will apply to most if not all MSRs modified on
context switch paths.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

tools/misc/xen-cpuid.c

@@ -189,6 +189,8 @@ static const char *const str_7a1[32] =
 
     [10] = "fzrm",          [11] = "fsrs",
     [12] = "fsrcs",
+
+    /* 18 */                [19] = "wrmsrns",
 };
 
 static const char *const str_e21a[32] =