[discussion] Deprecate `//cpu:armv8-m` in favor of `//cpu:armv8-m.main`

Similar to #100, I'm looking to tidy up some pre-existing Cortex-M CPU expressions here.

# Proposal
Mark `//cpu:armv8-m` as deprecated, and introduce `//cpu:armv8-m.base` and `//cpu:armv8-m.main`.

# Why?
ARMv8-M is technically split into two distinctly different ISAs: `armv8-m.main` and `armv8-m.base`. The baseline variant of ARMv8-M is more akin to `ARMv6-M` than `ARMv7E-M`. Also, when spelling `-march`, there is no such option for `armv8-m`. The flavors are either `armv8-m.base` or `armv8-m.main`. Note that these flavors are not traditionally considered extensions (unlike FPU or DSP instructions).

Also, this naming carries forward with the `armv8.1-m.main` architecture, even though it doesn't have a `armv8.1-m.base` variant.