std.builtin.StackTrace: format() workaround on freestanding

This whole thing needs to be reworked but for now at least don't cause a
compile error when building for a target that doesn't have stderr or
detectTTYConfig.

lib/std/builtin.zig

@@ -61,6 +61,11 @@ pub const StackTrace = struct {
         options: std.fmt.FormatOptions,
         writer: anytype,
     ) !void {
+        // TODO: re-evaluate whether to use format() methods at all.
+        // Until then, avoid an error when using GeneralPurposeAllocator with WebAssembly
+        // where it tries to call detectTTYConfig here.
+        if (builtin.os.tag == .freestanding) return;
+
         _ = fmt;
         _ = options;
         var arena = std.heap.ArenaAllocator.init(std.heap.page_allocator);