CI: download gcc

Created by: bluewww

Until now we always built gcc on travis-ci and cached it for some time. Building gcc is takes like ~1h of time and sometimes breaks or times out. I already contacted travis-ci once to increase our maximum allowed built time, but now gcc builds break because the pinned hash is no reachable anymore.

We now rely on pulp-gcc github releases which I made here https://github.com/pulp-platform/pulp-riscv-gnu-toolchain/releases which we simply wget in the setup phase of our test instance. This should result in less intermittent failures.