Hot-swap compiler versions

[nathaniel-brough]

Currently, the compiler version is hardcoded. This should be a configurable attribute so that users can bring their own compiler.

[tpudlik]

This would be very helpful to us on the Pigweed side, as it would allow us to bring the toolchains we use in gn and bazel in line (https://bugs.chromium.org/p/pigweed/issues/detail?id=637).