[fidl][llcpp] Generate llcpp bindings at build time

This removes the pre-building of llcpp bindings and instead builds them
as required at build time. This updates the zircon fidl llcpp build
rules to use fidlgen_llcpp rather than just pointing to the prebuilts.

Bug: 3240

Change-Id: I0902cdce98ddc460bd3bf49ff6786608c029b7da