feat(Other): Enable building zephyr for risc-v targets

[MaureenHelm]

Configures the zephyr build system to use the appropriate cmsis system startup and peripheral driver sources based on the target core architecture. The mxc lock driver is not implemented for risc-v and emits a build warning (which twister promotes to an error), therefore the zephyr build system is configured to build it only for arm.

Before this patch, we could only build zephyr for arm targets, but now we can build zephyr for arm or risc-v targets.

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[MaureenHelm]

@Jake-Carter can you take a look?

[Jake-Carter]

On RISC-V MXC_Lock implementation - we've had this in the TODO for a while. Let me know if we need to bump this up in priority.