objtool: Fix error handling for STD/CLD warnings

Actually return an error (and display a backtrace, if requested) for
directional bit warnings.

Fixes: 2f0f9e9a ("objtool: Add Direction Flag validation")
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lore.kernel.org/r/dc70f2adbc72f09526f7cab5b6feb8bf7f6c5ad4.1611263461.git.jpoimboe@redhat.com