[wasm] Use __visibility__("default") for all wasm platforms

The condition previously depended on the specific OS, but the image
boundary is the same for all platforms using Wasm as image format. This
fixes the Emscripten build.

Author: kateinoigakukun

stdlib/public/SwiftShims/swift/shims/Visibility.h

@@ -134,7 +134,7 @@
 // right for Windows, we have everything set up to get it right on
 // other targets as well, and doing so lets the compiler use more
 // efficient symbol access patterns.
-#if defined(__MACH__) || defined(__wasi__)
+#if defined(__MACH__) || defined(__wasm__)
 
 // On Mach-O and WebAssembly, we use non-hidden visibility.  We just use
 // default visibility on both imports and exports, both because these