x86/entry: Use 32bit XOR in handle_ist_exception

This is a micro-optimsiation for Silvermont microarchitectures, which don't
recognise the 64bit form as a zeroing idiom.

No functional change.

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

xen/arch/x86/x86_64/entry.S

@@ -1027,7 +1027,7 @@ handle_ist_exception:
          * Interrupted guest context. Clear the restore value for xen_cr3
          * and copy the context to stack bottom.
          */
-        xor   %r15, %r15
+        xor   %r15d, %r15d
         xor   %ebx, %ebx
         GET_CPUINFO_FIELD(guest_cpu_user_regs,di)
         movq  %rsp,%rsi