rust-target-config: match riscv target names with what rust expects

Official rust risc-v targets are prefixed with riscv32gc- and riscv64gc-:
https://doc.rust-lang.org/nightly/rustc/platform-support.html

Particularly crossbeam-utils make important build time decisions
for atomics based on those names, and so we need to match ours
with official targets.

On the other hand, the actual definitions for those targets do not
use the 'gc' suffix in 'arch' and 'llvm-target' fields, and so we
need to follow that too, to avoid cryptic mismatch errors from rust-llvm:
https://github.com/rust-lang/rust/blob/master/compiler/rustc_target/src/spec/riscv32gc_unknown_linux_gnu.rs



Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>