Default to double-float ABI on RISC-V Linux

[msizanoen1]

This is the default ABI used by GCC and rustc on linux (https://github.com/rust-lang/rust/blob/4007d4ef26eab44bdabc2b7574d032152264d3ad/src/librustc_target/spec/riscv64gc_unknown_linux_gnu.rs#L20).

[msizanoen1]

Also RISC-V ELF psABI documentation says that the default ABI is always double-float: https://github.com/riscv/riscv-elf-psabi-doc/blob/master/riscv-elf.md#default-abis
Maybe we can only use soft-float ABI for *-none-* target?

[msizanoen1]

Also there is riscv64gc-unknown-none-elf target and riscv32imafdc-unknown-none-elf target that supports hard float. Should we default to double-float ABI for these target in rustc?

[alexcrichton]

I'm not personally too familiar with the various naming conventions around the riscv targets, but I'm happy to merge PRs to incrementally improve support!

[msizanoen1]

Now a new cc version should be released to crates.io?

[alexcrichton]

Sure thing, done now!