rust-target-config.bbclass: Update for new riscv TUNE_FEATURES

rust-target-config.bbclass: Update for new riscv TUNE_FEATURES

Add the new TUNE_FEATURES to the 'features:' list, based on matching output
with:

   rustc --target=riscv32i-unknown-none-elf -Ctarget-feature=help

Use the TUNE_RISCV_ABI instead of guessing for the ABI.

Pass the arch "as-is", since it should now be riscv32 or riscv64.

Signed-off-by: Mark Hatle <mark.hatle@amd.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>