wasm: enable WAVM's stack unwinding feature

[mathetake]

This PR enables WAVM's stack unwind feature, which enhance WASM extension developers debuggability dramatically.
Without WAVM_ENABLE_UNWIND=on, the returned call stack is always empty as you can see here.

The corresponding PR in proxy-wasm-cpp-host is here: proxy-wasm/proxy-wasm-cpp-host#84

The stack trace message is something like this:

[2020-10-28 14:24:10.551][979699][trace][wasm] [source/extensions/common/wasm/wasm_vm.cc:29] Function proxy_on_tick failed:
WAVM message: wavm.reachedUnreachable
wasm backrace:
  1: wasm!helloworld!runtime._panic+7
  2: wasm!helloworld!command-line-arguments.C+2
  3: wasm!helloworld!command-line-arguments.B+0
  4: wasm!helloworld!command-line-arguments.A+0
  5: wasm!helloworld!(*command-line-arguments.helloWorld).OnTick+74
  6: wasm!helloworld!proxy_on_tick+0

Signed-off-by: mathetake takeshi@tetrate.io

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cc @lizan @PiotrSikora

[PiotrSikora]

@mathetake in the future, you might want to include proxy-wasm-cpp-host update in bazel/repository_locations.bzl that points to your fork while the proxy-wasm-cpp-host PR is still under review. This way, the CI tests the complete change, instead of only the Envoy part, and it signals that this PR is still blocked on the proxy-wasm-cpp-host PR.

In this particular case, the merged PR is a bit useless by itself.