arm: make return_address available for ARM_UNWIND
This fixes a warning saying:

    warning: #warning "TODO: return_address should use unwind tables"

And, this enables return_address using unwind information. If ARM_UNWIND is
selected, unwind_frame in unwind.c will be called in walk_stackframe.

Signed-off-by: sahara <keun-o.park@windriver.com>