travis: fetch xPack risc-v toolchain
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 <d7e19930cc1f42c2d0781f4d9e6f1fe5891bf9cf@xobs.io>

hathach/tinyusb,hathach/tinyusb,hathach/tinyusb