Provide (undocumented for now) __pause() call to emit PAUSE inst.

builtins/util.m4

@@ -1603,6 +1603,11 @@ declare void @ISPCInstrument(i8*, i8*, i32, i64) nounwind
 declare i1 @__is_compile_time_constant_mask(<WIDTH x MASK> %mask)
 declare i1 @__is_compile_time_constant_varying_int32(<WIDTH x i32>)
 
+define void @__pause() nounwind readnone {
+  call void asm sideeffect "pause", "~{dirflag},~{fpsr},~{flags}"() nounwind
+  ret void
+}
+
 ; This function declares placeholder masked store functions for the
 ;  front-end to use.
 ;