Error building for riscv64gc

[banditopazzo]

if you build for riscv64gc you get an error from bindgen:

 error: unknown target triple 'riscv64gc-unknown-linux-gnu', please use -triple or -arch
  thread 'main' panicked at 'libclang error; possible causes include:
  - Invalid flag syntax
  - Unrecognized flags
  - Invalid flag arguments
  - File I/O errors
  - Host vs. target architecture mismatch