Makefile: link arch/ last

The IRQ implementations of ARC, ARM, and Nios II use .gnu.linkonce
sections for declaring entires in the sw_isr_table array. It's
imperative that arch/built-in.o be linked after everything else
as we want custom interrupt handlers to take precedence over the
default spurious interrupt handlers.

Currently, any interrupt handlers defined in ext/ or usb/ will not
be installed properly on the above mentioned arches.

Change-Id: Ib3fb21ff1ef114678906f130c268c266535954f1
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>