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.

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);
         }