travis: fetch xPack risc-v toolchain

This toolchain seems popular in the embedded space, and is generally
preferred over the upstream SiFive toolchain.  It can produce both
32- and 64-bit binaries, so its prefix is riscv-none-embed-.

Signed-off-by: Sean Cross <sean@xobs.io>

.travis.yml

@@ -16,7 +16,11 @@ install:
   - gem install ceedling
       
 before_script:
+  - wget -O /tmp/riscv-toolchain.tgz https://github.com/xpack-dev-tools/riscv-none-embed-gcc-xpack/releases/download/v8.3.0-1.1/xpack-riscv-none-embed-gcc-8.3.0-1.1-linux-x64.tgz
+  - tar -xzf /tmp/riscv-toolchain.tgz
+  - export PATH=$PWD/xPacks/riscv-none-embed-gcc/8.3.0-1.1/bin:$PATH
   - arm-none-eabi-gcc --version
+  - riscv-none-embed-gcc --version
 
 script:
   # Build all examples