tests/multiboot: Fix load address of test kernels

tests/multiboot: Fix load address of test kernels

While older toolchains produced binaries where the physical load address
of ELF segments was the same as the virtual address, newer versions seem
to choose a different physical address if it isn't specified explicitly.
The means that the test kernel doesn't use the right addresses to access
e.g. format strings any more and the whole output disappears, causing
all test cases to fail.

Fix this by specifying the physical load address of sections explicitly.

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