objtool: Clean up compiler flag usage

objtool: Clean up compiler flag usage

KBUILD_HOSTCFLAGS and KBUILD_HOSTLDFLAGS aren't defined when objtool is
built standalone.  Also, the EXTRA_WARNINGS flags are rather arbitrary.

Make things simpler and more consistent by specifying compiler flags
explicitly and tweaking the warnings.  Also make a few code tweaks to
make the new warnings happy.

Acked-by: Petr Mladek <pmladek@suse.com>
Tested-by: Joe Lawrence <joe.lawrence@redhat.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>