fix 64-bit issues in the garbage collection

We discovered that if a single Erlang process tried to grow above 32
GB (i.e., more 64-bit words than can be counted by a 32-bit number),
the VM failed to find the next larger heap size, even though there
were plenty more heap sizes left to pick from and even though we had a
lot more memory available on the machine. (Obviously, this is only
applicable on 64-bit Erlang.)

It turned out to be due to some 'int' variables in the heap resizing
parts of erl_gc.c not being properly updated to 'Uint' or 'Sint'. Once
that was fixed, I got segfaults instead as soon as the heap got larger
than 2^32 words, due to even more 'int' declarations in the same file,
but now in the GC code.

After fixing this as well, I successfully ran an Erlang node in which
a single Erlang process had a heap so large that I'm not at liberty to
divulge the exact size, but I think the scientific term is
"humongous", and I'm confident that there are no further immediate
problems with very very large individual process heaps.

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