Skip building thrinfo_t tree when mt is disabled.

Details:
- Return early from bli_thrinfo_sup_grow() if the thrinfo_t object
  address is equal to either &BLIS_GEMM_SINGLE_THREADED or
  &BLIS_PACKM_SINGLE_THREADED.
- Added preprocessor logic to bli_l3_sup_thread_decorator() in
  bli_l3_sup_decor_single.c that (by default) disables code that
  creates and frees the thrinfo_t tree and instead passes
  &BLIS_GEMM_SINGLE_THREADED as the thrinfo_t pointer into the
  sup implementation.
- The net effect of the above changes is that a small amount of
  thrinfo_t overhead is avoided when running small/skinny dgemm
  problems when BLIS is compiled with multithreading disabled.

frame/thread/bli_thrinfo_sup.c

@@ -42,6 +42,9 @@ void bli_thrinfo_sup_grow
        thrinfo_t* thread
      )
 {
+	if ( thread == &BLIS_GEMM_SINGLE_THREADED ||
+	     thread == &BLIS_PACKM_SINGLE_THREADED ) return;
+
 	// NOTE: If bli_thrinfo_sup_rgrow() is being called, the sub_node field will
 	// always be non-NULL, and so there's no need to check it.
 	//if ( bli_cntl_sub_node( cntl ) != NULL )