objtool: make it visible in make V=1 output

It is currently impossible to see what is going on with objtool when
building, so call echo-cmd to see the actions:
  gcc -Wp,-MD,arch/x86/entry/.entry_64.o.d  -nostdinc -isystem ...
  ./tools/objtool/objtool check "arch/x86/entry/entry_64.o";

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Michal Marek <mmarek@suse.com>
Cc: linux-kbuild@vger.kernel.org
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>