Fix logic when dealing with -a arch.target_arch or -a arch.

Allow the user to specify the -a argument multiple ways.

Prior to this, -a mips.mips would fail to find the nxb-bin dir in /usr/obj/mips.mips
TARGET being used by the buildworld and its not throwing an error now.  What I really wanted was TARGET_ARCH here for the copy of the XDEV_TOOLS dir for crossbuilding.  The use of TARGET in this function isn't clear.

Overriding TARGET may have been a mistake to begin with.