For the final step of the native-xtools target, "everything" is built
with TARGET and TARGET_ARCH set to the architecture we wish to
cross-build for. However, CROSSENV overwrites the values of MACHINE and
MACHINE_ARCH, setting them to be identical to TARGET and TARGET_ARCH.
For native-xtools this is undesirable since we are building binaries to
run on the host architecture, that can cross compile for the target
architecture.
When building native-xtools for RISC-V, this issue manifests as an
invalid argument for "-march", as the compiler is expecting an x86
processor, not rv64imafdc. These flags are set in bsd.cpu.mk depending
on MACHINE_CPUARCH, therefore demonstrating the need for MACHINE and
MACHINE_ARCH to be set to the host's architecture.