ensure thrinfo is valid at the end of bli_l3_sup_thread_decorator_entry

[chillenb]

This is an attempt to fix #919. I'm not too familiar with the codebase, so I'm not sure if the barrier at the end of bli_l3_sup_thread_decorator_entry needs to use the original thrinfo or after modification inside bli_gemmsup_int / bli_gemmtsup_int.

[chillenb]

I decided to do it a different way (less invasive, fewer changes)

1. Ensure bli_l3_sup_thrinfo_update(rntm, root) doesn't free root. Instead, it frees and rebuilds the subtree below root.
2. Change the signature

void bli_l3_sup_thrinfo_update
     (
       const rntm_t*     rntm,
             thrinfo_t** root
     );

to

void bli_l3_sup_thrinfo_update
     (
       const rntm_t*     rntm,
             thrinfo_t*  root
     );

to reflect the fact that root is not being freed.