add more Cortex-R targets

This expands on PR #53663 to complete the set of Cortex-R targets and builds
rust-std components for them.

r? @alexcrichton

each extra rust-std component (there's 4 of them) takes about 3 minutes to build
on my local machine. In terms of stability (LLVM codegen bugs) these new targets
should be as stable as the Cortex-M ones (e.g. `thumbv7m-none-eabi`).

If the extra build time is too much we can leave the rust-std components out for
now

closes #53663
cc @paoloteti

199: [RFC] initial Cortex-R team r=japaric a=japaric

This is a proposal for creating a Cortex-R team.

@paoloteti and I will be the initial members. I know pretty much nothing about
the Cortex-R but will be working with them starting November; @paoloteti is the
real expert here.

The following work has been planed for this team (some of it has been done
already)

- [x] Add more built-in targets to cover existing devices. In
  rust-lang/rust#53679 we went from 1 target to 4. We now have good coverage of
  Cortex R4(F) and R5(F) cores.

- [x] Move to LLD to remove the dependency on a external linker. Done in
  rust-lang/rust#53679.

- [x] Produce `rust-std` components to not depend on tools like Xargo, which
  require nightly. Also done in rust-lang/rust#53679.

- [ ] Create a `cortex-r` crate (\*) akin to the `cortex-m` crate that provides
  a safe API to emit arch-specific instructions and to manipulate system
  registers.

- [ ] Create a `cortex-r-rt` crate (\*) akin to the `cortex-m-rt` crate that
  helps building a minimal `no_std` binary.

- [ ] See if we can generate register APIs from the XML files that vendors like
  TI ship with their IDEs.

(\*) Initial work in this area: https://github.com/paoloteti/ti-hercules-bsp

@rust-embedded/all please vote on this proposal using [GitHub reviews][approve].
This proposal needs at least 11 approvals to land (/all has 21 members atm).

[approve]: https://help.github.com/articles/approving-a-pull-request-with-required-reviews/


Co-authored-by: Jorge Aparicio <jorge@japaric.io>