i#3544 RISC-V, part 13: Add cross config file

Created by: semihalf-kardach-stanislaw

This enables cross-compiling DynamoRIO for RISC-V using Ubuntu default toolchain prefix of riscv64-linux-gnu-. To cross-compile DynamoRIO for RISC-V do the following:

  # Install a cross toolchain.
  sudo apt install g++-riscv64-linux-gnu
  # From the root of DynamoRIO
  mkdir build-riscv && cd build-riscv
  cmake -DCMAKE_TOOLCHAIN_FILE=../make/toolchain-riscv64.cmake ..
  make -j

Issue: #3544 Depends: #5608, #5609, #5625

Signed-off-by: Stanislaw Kardach kda@semihalf.com