Add riscv64 support.

[JasenChao]

Built success in https://openkoji.iscas.ac.cn/koji/taskinfo?taskID=1285

[glaubitz]

Built success in https://openkoji.iscas.ac.cn/koji/taskinfo?taskID=1285

Can you explain how you managed to create the bindings using a current version of LLVM?

For me, the command failed with:

(sid_sparc64-dchroot)glaubitz@stadler:~/rust-lv2$ cargo run -p systool -- --lv2 /usr/include/lv2 --out sys/src/linux/sparc64.rs -- --target=sparc64-linux-gnu
    Finished dev [unoptimized + debuginfo] target(s) in 0.17s
     Running `target/debug/systool --lv2 /usr/include/lv2 --out sys/src/linux/sparc64.rs -- --target=sparc64-linux-gnu`
thread 'main' panicked at /home/glaubitz/.cargo/registry/src/index.crates.io-d11c229612889eed/bindgen-0.53.3/src/ir/context.rs:894:9:
"LV2_Atom_Event_union_(unnamed_at_/usr/include/lv2/atom/atom_h_203_3)" is not a valid Ident
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
(sid_sparc64-dchroot)glaubitz@stadler:~/rust-lv2$

[glaubitz]

Built success in https://openkoji.iscas.ac.cn/koji/taskinfo?taskID=1285

Can you explain how you managed to create the bindings using a current version of LLVM?

For me, the command failed with:

(sid_sparc64-dchroot)glaubitz@stadler:~/rust-lv2$ cargo run -p systool -- --lv2 /usr/include/lv2 --out sys/src/linux/sparc64.rs -- --target=sparc64-linux-gnu
    Finished dev [unoptimized + debuginfo] target(s) in 0.17s
     Running `target/debug/systool --lv2 /usr/include/lv2 --out sys/src/linux/sparc64.rs -- --target=sparc64-linux-gnu`
thread 'main' panicked at /home/glaubitz/.cargo/registry/src/index.crates.io-d11c229612889eed/bindgen-0.53.3/src/ir/context.rs:894:9:
"LV2_Atom_Event_union_(unnamed_at_/usr/include/lv2/atom/atom_h_203_3)" is not a valid Ident
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
(sid_sparc64-dchroot)glaubitz@stadler:~/rust-lv2$

OK, fixed by updating bindgen in sys/tool/Cargo.toml to 0.62.0.