Fix HTML5 lib LTO build with PTHREADS

We use WebGPU, pthread and PROXY_TO_PTHREAD and noticed a linker error in our release builds where we enable LTO:

wasm-ld: error: C:\\Dev\\emsdk\\upstream\\emscripten\\cache\\sysroot\\lib\\wasm32-emscripten\\lto\libhtml5.a(callback.o): attempt to add bitcode file after LTO (_emscripten_run_callback_on_thread)

Judging by other tickets this seems like the place to fix it, but let me know if that's wrong.

Author: ifiddynine

tools/system_libs.py

@@ -1036,7 +1036,11 @@ def get_libcall_files(self):
       path='system/lib/libc/musl/src/errno',
       filenames=['__errno_location.c'])
 
-    return math_files + exit_files + other_files + iprintf_files + errno_files
+    html5_files += files_in_path(
+      path='system/lib/html5',
+      filenames=['callback.c'])
+
+    return math_files + exit_files + other_files + iprintf_files + errno_files + html5_files
 
   def get_files(self):
     libc_files = []