Migrate runtime builds from Envoy repository to cpp-host

[mathetake]

Runtime build issues/handling should be decoupled from host implementations, e.g. the link time symbol issue envoyproxy/envoy#14012

TODOs:

• have BUILD files for WAVM and V8 (hopefully without gen_rules Migrate wee8 from genrule to BUILD file envoyproxy/envoy-wasm#206)
  • V8: https://github.com/envoyproxy/envoy/blob/master/bazel/external/wee8.BUILD
  • WAVM: https://github.com/envoyproxy/envoy/blob/master/bazel/foreign_cc/BUILD#L314-L342
• have exactly the same lib targest as in https://github.com/envoyproxy/envoy/blob/master/bazel/external/proxy_wasm_cpp_host.BUILD
  • ${runtime}_lib

Related to #49