Get rid of anonymous struct

Get rid of anonymous struct

This is a C11 feature, and we require C99. While at it, go the further
step and get rid of the surrounding union (with uintptr_t) entirely,
as there is currently no use case for this file to access the header of
BlocktableEntry as a uintptr_t, and there are no additional alignment
requirements. The least invasive way seems to be to transfer the old
union name to this struct.

Reported by Pavel Borisov and Andres Freund, per buildfarm member mylodon
Reviewed by Pavel Borisov

Discussion: https://postgr.es/m/CALT9ZEH11NYV8AOzKb1bWhCf6J0H=H31f0MgT9xX+HdqvcA1rw@mail.gmail.com