objtool: Use arch specific values in restore_reg()

The initial register state is set up by arch specific code. Use the
value the arch code has set when restoring registers from the stack.

Suggested-by: Raphael Gault <raphael.gault@arm.com>
Signed-off-by: Julien Thierry <jthierry@redhat.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Miroslav Benes <mbenes@suse.cz>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>