RISC-V: Re-enable GCC+Rust builds

[Asuna]

> For example, there's a check in the riscv Kconfig menu to see if 
> stack-protector-guard=tls can be used via a cc-option check. If that 
> check passes with gcc as the compiler that option will be passed to 
> the rust side of the build, where llvm might not support it.
If I understand correctly, the `-mstack-protector-guard` option is 
already always filtered out by `bindgen_skip_c_flags` in 
`rust/Makefile`, regardless of architecture. Therefore, we don't need to 
do anything more, right?

> Similarly, turning on an extension like Zacas via a cc-option check 
> could pass for gcc but not be usable when passed to the rust side, 
> causing errors.
That makes sense. I might need to check the version of libclang for each 
extension that passes the cc-option check for GCC to ensure it supports 
them.

> These sorts of things should be prevented via Kconfig, not show up as 
> confusing build errors.
I'm working on a patch, and intend to output an error message in 
`arch/riscv/Makefile` then exit 1 when detecting an incompatible 
gcc+libclang mix in use.