ABI mismatch for riscv64gc-unknown-none-elf

[dramforever]

Currently cc builds for riscv*-unknown-none-elf targets with -mabi=lp64:

cc-rs/src/lib.rs

Lines 1973 to 1975 in 06c1289

	} else if arch.starts_with("64") {
	cmd.args.push(("-march=rv".to_owned() + arch).into());
	cmd.args.push("-mabi=lp64".into());

However for riscv64gc-unknown-none-elf rustc produces lp64d binaries, which would fail to link with lp64 binaries. Adding .flag("-mabi=lp64d") works around this issue.

See this demo repo: https://github.com/dramforever/demo-cc-rs-riscv64gc

$ cargo build --target riscv64gc-unknown-none-elf
   Compiling demo-cc-rs-riscv64gc v0.1.0 (/home/dram/code/virtiotic/demo-cc-rs-riscv64gc)
error: linking with `rust-lld` failed: exit status: 1
  |
  = note: [omitted]
  = note: rust-lld: error: /home/dram/code/virtiotic/demo-cc-rs-riscv64gc/target/riscv64gc-unknown-none-elf/debug/build/demo-cc-rs-riscv64gc-add89ab8ebfbee8b/out/libanswer.a(answer.o): cannot link object files with different floating-point ABI
          

error: could not compile `demo-cc-rs-riscv64gc` due to previous error

Maybe we should copy the default ABI settings from rustc instead of using the current detection scheme. Unfortunately I couldn't find any docs on this.