bootstrap.sh should mention that musl also has arch-aware build rules to update when adding a new arch

scripts/bootstrap.sh provides a list of packages with architecture-aware build rules that need to be updated prior to bootstrapping a new architecture. That list is currently missing musl, which also needs such an update to architecture detection in build rules. If this is not done by the ambitious developer trying to get Alpine going on a new architecture, the attempt by bootstrap to pull in musl headers via musl-dev prior to GCC stage 2 build will fail with a misleading error about a missing directory under the musl port.