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.