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,

src/codegen/llvm.zig

@@ -9525,7 +9525,7 @@ pub const FuncGen = struct {
         _ = inst;
         const o = self.ng.object;
         const llvm_usize = try o.lowerType(Type.usize);
-        if (!target_util.supportsReturnAddress(o.pt.zcu.getTarget())) {
+        if (!target_util.supportsReturnAddress(o.pt.zcu.getTarget(), self.ng.ownerModule().optimize_mode)) {
             // https://github.com/ziglang/zig/issues/11946
             return o.builder.intValue(llvm_usize, 0);
         }

src/target.zig

@@ -248,9 +248,14 @@ pub fn libcProvidesStackProtector(target: std.Target) bool {
     return !target.isMinGW() and target.os.tag != .wasi and !target.cpu.arch.isSpirV();
 }
 
-pub fn supportsReturnAddress(target: std.Target) bool {
+/// Returns true if `@returnAddress()` is supported by the target and has a
+/// reasonably performant implementation for the requested optimization mode.
+pub fn supportsReturnAddress(target: std.Target, optimize: std.builtin.OptimizeMode) bool {
     return switch (target.cpu.arch) {
-        .wasm32, .wasm64 => target.os.tag == .emscripten,
+        // 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.
+        .wasm32, .wasm64 => target.os.tag == .emscripten and optimize == .Debug,
         .bpfel, .bpfeb => false,
         .spirv, .spirv32, .spirv64 => false,
         else => true,