Lower @returnAddress to a constant 0 in Emscripten release builds

Emscripten currently implements `emscripten_return_address()` by calling
out into JavaScript and parsing a stack trace, which introduces
significant overhead that we would prefer to avoid in release builds.

This is especially problematic for allocators because the generic parts
of `std.mem.Allocator` make frequent use of `@returnAddress`, even
though very few allocator implementations even observe the return
address, which makes allocators nigh unusable for performance-critical
applications like games if the compiler is unable to devirtualize the
allocator calls.

lib/std/debug.zig

@@ -183,9 +183,11 @@ pub const sys_can_stack_trace = switch (builtin.cpu.arch) {
 
     // `@returnAddress()` in LLVM 10 gives
     // "Non-Emscripten WebAssembly hasn't implemented __builtin_return_address".
+    // On Emscripten, Zig only supports `@returnAddress()` in debug builds
+    // because Emscripten's implementation is very slow.
     .wasm32,
     .wasm64,
-    => native_os == .emscripten,
+    => native_os == .emscripten and builtin.mode == .Debug,
 
     // `@returnAddress()` is unsupported in LLVM 13.
     .bpfel,