x86/kexec: Make relocate_kernel_64.S objtool clean

Having fixed the biggest objtool issue in this file; fix up the rest
and remove the exception.

Suggested-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Miroslav Benes <mbenes@suse.cz>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lkml.kernel.org/r/20200324160924.202621656@infradead.org