Support future riscv64-unknown-linux-elf target

[fintelia]

Rust currently has support for bare metal RISC-V targets, but cannot compile for RISC-V Linux ones because it lacks standard library support. This is partially blocked because the libc crate does not recognize target_arch = "riscv64" and thus does not export symbols like c_int, c_void, and uintptr_t for it.

[gnzlbg]

A good place to start would be by adding a docker container for the target, e.g., that install gcc-riscv64-linux-gnu libc6-dev-riscv64-cross qemu-system-riscv64, and compiles libc for the target. That should not be blocked by anything.

If compilation fails, that container can be added to allow_failures: in the .travis.yml.