[riscv] Enable More RISC-V Targets (#2326)

RISC-V became a default backend in Clang 9.0. This commit enables the
use of the RISC-V backend for Clang in versions:
- 9.0.0
- 10.0.0
- 10.0.1
- 11.0.0

I also did a few other tidy-ups:
- I ensured that these could use more headers for rv64, by adding the
  `--gcc-toolchain` and `--sysroot` args to point to the rv64 8.2.0
  cross-compiled gcc toolchain.
- I updated the `-mabi` for rv64gc so it matches what that toolchain is
  likely to have chosen as a default (and what a user is likely to
  expect with the gc extension). I should check this, but I'm not sure
  how I can.
- I updated the `-mabi` for rv32gc so it matches what a user is likely
  to choose by default (ilp32d).
- I changed the config keys to introduce a hyphen, to match the arm
  compiler keys (and added the right alias).
- I'm not sure if older versions of CE-built clang also have RISC-V,
  given that the clang-builder repo has `RISCV` in the experimental
  targets to build. (PR incoming to remove that, as it's no longer
  needed).

I haven't been able to test this, partially because it depends on the
exact compiler-explorer setup on amazon, but I gleaned these details
from code in the compiler-explorer/infra repository.

Signed-off-by: Sam Elliott <sam@lenary.co.uk>

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