Makefiles: Remove the executable bit from the .elf and .flat files

The .elf and .flat files are not runnable on the host operating system,
so they should not be marked as executable.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1532702645-30780-1-git-send-email-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by:
Acked-by: Andrew Jones <drjones@redhat.com>