tests/boot-sector: Do not overwrite the x86 buffer on other architectures

Re-using the boot_sector code buffer from x86 for other architectures
is not very nice, especially if we add more architectures later. It's
also ugly that the test uses a huge pre-initialized array at all - the
size of the executable is very huge due to this array. So let's use a
separate buffer for each architecture instead, allocated from the heap,
so that we really just use the memory that we need.

Suggested-by: Michael Tsirkin <mst@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Message-Id: <1502431076-22849-2-git-send-email-thuth@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
