tests: Multiboot mmap test case

This adds a test case for Multiboot memory map in the tests/multiboot
directory, where future i386 test kernels can be dropped. Because this
requires an x86 build host and an installed 32 bit libgcc, the test is
not part of a regular 'make check'.

The reference output for the test is verified against test runs of the
same multiboot kernel booted by some GRUB 0.97.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>