RISC-V: Re-enable GCC+Rust builds

[Asuna]

(I apologize if anyone gets this email twice, the first time I 
mistakenly sent it as HTML and it was rejected by mailing lists)

> Similarly, something like -Wno-unterminated-string-initialization could
> cause a problem if gcc supports it but not libclang.
And the -Wno-unterminated-string-initialization is not supposed to be a 
problem either, today I noticed that in rust/Makefile there is:

    # All warnings are inhibited since GCC builds are very experimental,
    # many GCC warnings are not supported by Clang, they may only appear in
    # some configurations, with new GCC versions, etc.
    bindgen_extra_c_flags = -w --target=$(BINDGEN_TARGET)

The -w flag inhibits all warnings, even though Clang may not recognize 
it. I was not able to reproduce any errors related to this.

I have a patch ready and will send it later. Please let me know if I'm 
missing something there. Thanks.