This needs to be aligned with some changes to gdb to consume the dumps
it produces, so it's probably best to hold off applying this until it's
more obvious what's going to happen with those.
Random notes:
- objdump identifies the output of dumper on x86_64 as
'elf64-x86-64-cloudabi' (perhaps due to some over-eager sniffer).
- regions excluded from the dump aren't rounded up to page size, so we
may end up writing the excess into the dump.
- looking at the loaded modules and inspecting them to determine what
memory regions don't need to appear in the dump seems odd. I'm not sure
we don't just exclude MEMORY_BASIC_INFORMATION.Type == MEM_IMAGE regions
(assuming they get converted to MEM_PRIVATE regions if written when
copy-on-write).
