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
I'll put back this line as I don't know what its being used for.  :-)

I'll also change the copy of XDEV_TOOLS to use TARGET.TARGET_ARCH