Ugh this issue is deeply frustrating, the issue is that building requires both compiling tools for the host machine x86_64-unknown-linux-gnu and libraries for the target armv6m-none-none-eabi but -Xcc applies to both the host and the target.
@xedin had a pitch to improve this situation: Re-examining meaning of global options in cross-compilation contexts
I'm not sure where this went and I think its a no-brainer.
cc @dschaefer2