ARC: module: Fix !CONFIG_ARC_DW2_UNWIND builds

commit d65283f7 added mod->arch.secstr under
CONFIG_ARC_DW2_UNWIND, but used it unconditionally which broke builds
when the option was disabled. Fix that by adjusting the #ifdef guard.

And while at it add a missing guard (for unwinder) in module.c as well

Reported-by: Waldemar Brodkorb <wbx@openadk.org>
Cc: stable@vger.kernel.org    #4.9
Fixes: d65283f7 ("ARC: module: elide loop to save reference to .eh_frame")
Tested-by: Anton Kolesov <akolesov@synopsys.com>
Reviewed-by: Alexey Brodkin <abrodkin@synopsys.com>
[abrodkin: provided fixlet to Kconfig per failure in allnoconfig build]
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>