Reduce memory allocations and simplify the mbmem loop

[Schwarzemann]

The collect_multiboot_info function allocates memory for mmap using malloc. Depending on the size of the multiboot_mmap array, this could be a lot of memory. Instead, a stack-allocated array or a pre-allocated buffer to store the mmap data would work perfectly and the mbmem loop that converts the mmap records is somewhat complex and difficult to follow.

[mcayland]

Thanks for the patch! Whilst the repositories are hosted on github, the project still makes use of mailing lists for patches/code review. If you can resend the patch to the OpenBIOS mailing list (https://mail.coreboot.org/postorius/lists/openbios.openbios.org/) we should be able to get this reviewed fairly quickly.

One question: do you have a working test case for amd64? I've got tests for all the other archs, but it would give me more confidence when reviewing the patch if I have a way to functionally verify it.