native-xtools/xdev: Store these in a location including both MACHINE/TARGET.

Given MACHINE/MACHINE_ARCH=amd64, TARGET=arm64 and TARGET_ARCH=aarch64,
this change will build them in a location such as:

/usr/obj/usr/src/amd64.amd64/nxb/arm64.aarch64/

and

/usr/obj/usr/src/amd64.amd64/xdev/aarch64-freebsd/

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