WIP: Make Int64#/Word64 and Int32#/Word32# useful on all architectures

After rearranging things to make a more readable diff, I add some primpos for these, and switch boxed Int64/Word64 to always use Int64#/Word64#. The smaller boxed number types (8, 16, and 32 bits) still used Int# for now; that can be changed later.

This is much more straightforward for downstream than my earlier attempt to excise MachDeps.h from primtops.txt.pp.

Fixes #11953 (closed), #17377 (closed), #17375 (closed)

CC @carter

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