This subcommand has been broken for a while, since libraries also use
the CPU flag. This commit fixes this.

Previously, libraries were usable for most Cortex-M cores. But with the
addition of the CPU field, I've limited it to three popular cores: the
Cortex-M0 (microbit), Cortex-M0+ (atsamd21), and Cortex-M4 (atsamd21,
nrf52, and many others).

In the future we might consider also building libraries for the current
OS/arch so that libraries like musl are already precompiled.