New RISC-V port (#281)

New RISC-V port (#281)

* Add RISC-V support

This patch adds support for the RISC-V architecture (https://riscv.org).

This patch has been tested using QEMU user-mode emulation and GCC 7.2.0
in the following configurations:

* -march=rv32imac -mabi=ilp32
* -march=rv32g -mabi=ilp32d
* -march=rv64imac -mabi=lp64
* -march=rv64g -mabi=lp64d

The ABI currently can be found at
https://github.com/riscv/riscv-elf-psabi-doc/blob/master/riscv-elf.md .

* Add RISC-V to README

* RISC-V: fix configure.host