Replace GC macro with function call
Replace GC macro with function call

CPython 3.7.3->3.7.4 changed the size of PyGC_Head, the macro
_PyObject_GC_UNTRACK relied on calling sizeof() on that struct,
as it is a macro it got baked in at compile time and then
segfaults happen across the point version change as the struct
size changed.

seibert/numba,seibert/numba,IntelLabs/numba,gmarkall/numba,stonebig/numba,seibert/numba,sklam/numba,cpcloud/numba,numba/numba,numba/numba,stonebig/numba,stuartarchibald/numba,cpcloud/numba,IntelLabs/numba,cpcloud/numba,IntelLabs/numba,IntelLabs/numba,seibert/numba,numba/numba,IntelLabs/numba,numba/numba,sklam/numba,gmarkall/numba,numba/numba,sklam/numba,seibert/numba,stonebig/numba,cpcloud/numba,sklam/numba,stuartarchibald/numba,gmarkall/numba,gmarkall/numba,stonebig/numba,stonebig/numba,cpcloud/numba,stuartarchibald/numba,stuartarchibald/numba,sklam/numba,gmarkall/numba,stuartarchibald/numba