Set no-op backtrace for all wasm family targets

[arjunr2]

Particularly needed to support wasm32-wali-linux-musl appropriately

[workingjubilee]

it is not clear to me that backtraces should be no-op for all wasm-family targets|

[arjunr2]

Backtraces aren't supported on any wasm target besides those that use emscripten right now; I figured this should be reflected. Would you rather I gate it on just not(all(target_arch = wasm32, target_os = linux))?

[workingjubilee]

My understanding, and it could be wrong, is the current environs is that one or two wasm targets are moving towards unwinding support, where it seems better to have noisier failures when they try to build and this code starts mattering than a silent failure to produce traces.

And that everyone else who isn't, is just building with panic="abort" so it doesn't matter to them?