gnu: Add arm-none-eabi-toolchain variants.

* gnu/packages/embedded.scm (arm-none-eabi-toolchain): New procedure.
(arm-none-eabi-toolchain-4.9, arm-none-eabi-nano-toolchain-4.9,
arm-none-eabi-toolchain-6, arm-none-eabi-nano-toolchain-6): New
variables.

Co-authored-by: Ricardo Wurmus <rekado@elephly.net>