NEW: devel/riscv-elf

This port is a cross-toolchain for RISC-V architecture which is based on
devel/arm-none-eabi port. On i386 and amd64, riscv64-unknown-elf-gcc can
build an executable for 32-bit and 64-bit targets that spike [1] + pk [2]
will run on Linux.

Comments/OKs ?

[1] https://github.com/riscv/riscv-isa-sim
[2] https://github.com/riscv/riscv-pk