Missing RISCV support

[XvR-Technolution]

Currently, RISCV support is missing. The target is not recognized and the system compiler is invoked, which will of course fail. Since RISCV is an supported RUST target in stable, I think it would make sense to support this platform.
For my current project on RISCV32, I could add support (at least for GCC) by simply adding:

"riscv32imac-unknown-none-elf" => Some("riscv32-unknown-elf"),

in src/lib.rs:Build::get_base_compiler (around line 1800). I guess for complete support you should add similar lines for:

$ rustup run stable rustc --print target-list | grep risc
riscv32imac-unknown-none-elf
riscv32imc-unknown-none-elf
riscv64gc-unknown-none-elf
riscv64imac-unknown-none-elf

Which would probably be something like:

"riscv32imac-unknown-none-elf" => Some("riscv32-unknown-elf"),
"riscv32imc-unknown-none-elf" => Some("riscv32-unknown-elf"),
"riscv64gc-unknown-none-elf" => Some("riscv64-unknown-elf"),
"riscv64imac-unknown-none-elf" => Some("riscv64-unknown-elf"),

Please note that I didn't test this!