Fix wasm32-unknown-emscripten cross-compile

[kofigumbs]

This PR adds workarounds for two issues that manifest when compiling for WASM with Emscripten:

• Bindgen doesn't always detect the correct target. This is marked as a bug upstream, so hopefully this workaround is temporary. On macOS, this manifests as the following error:

  --- stderr
  /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/sys/cdefs.h:828:2: error: Unsupported architecture
  /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/machine/_types.h:36:2: error: architecture not supported
  /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/sys/_types.h:55:9: error: unknown type name '__int64_t'
...

• Clang assumes the wrong visibility when building WASM targets. This manifests as a series of cannot find function errors:

   Compiling miniaudio v0.10.0 (/Users/kofi/Documents/miniaudio-rs/miniaudio)
error[E0425]: cannot find function `ma_context_init` in crate `sys`
  --> miniaudio-sys/examples/simple-enumeration-raw.rs:13:13
   |
13 |     if sys::ma_context_init(null(), 0, null(), context.as_mut_ptr()) != sys::MA_SUCCESS as _ {
   |             ^^^^^^^^^^^^^^^ not found in `sys`

error[E0425]: cannot find function `ma_context_get_devices` in crate `sys`
  --> miniaudio-sys/examples/simple-enumeration-raw.rs:25:23
   |
25 |     let result = sys::ma_context_get_devices(
   |                       ^^^^^^^^^^^^^^^^^^^^^^ not found in `sys`