objtool: Use O_CREAT with explicit mode mask

Recent Ubuntu enforces 3-argument open() with O_CREAT:

      CC      /home/mingo/tip/tools/objtool/builtin-check.o
    In file included from /usr/include/fcntl.h:341,
                     from builtin-check.c:9:
    In function ‘open’,
        inlined from ‘copy_file’ at builtin-check.c:201:11:
    /usr/include/x86_64-linux-gnu/bits/fcntl2.h:52:11: error: call to ‘__open_missing_mode’ declared with attribute error: open with O_CREAT or O_TMPFILE in second argument needs 3 arguments
       52 |           __open_missing_mode ();
          |           ^~~~~~~~~~~~~~~~~~~~~~

Use 0400 as the most restrictive mode for the new file.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org