Fix nrt debug print - #7027

This PR is a byproduct of me trying to fix generators. Without this PR I get output like this

*** NRT_Decref 140720103881220 [000002C1CB230770]
*** NRT_Incref 29778180 [000002C1CB230770] 
*** NRT_Decref 140720103881221 [000002C1CB230770] 

This is because the call to printf was essentially

printf("*** NRT_Decref %zu [%p]\n", *ptr, ptr)

The problem here is that the type of ptr is int8*., meaning the a value of type int8 was passed to printf when it expected a value of type uint64. This is undefined behaviour and apparently didn't break for others. I now changed it to be:

printf("*** NRT_Decref %zu [%p]\n", *((uint64*) ptr), ptr)

which does work correctly for me.

At request of @gmarkall I also added a config variable 'NUMBA_DEBUG_NRT' to replace the global _debug_print in nrtdynmod