Changes descriptions:

• The glib_flavor == "upstream" path unconditionally creates an arm64 output group that triggers a separate meson configure for building agent/gadget binaries, which tries to download the prebuilt toolchain from build.frida.re. Even with compat=disabled, this code path runs.
• When state.allowed_prebuilds is an empty set (from --without-prebuilds), the code overrides allowed to None, re-enabling all prebuild downloads.
• Android Bionic has opaque FILE* struct (like musl), but the code only guards with #ifdef HAVE_MUSL. Accessing FILE->_lbf_size etc. fails on Bionic.
• Bionic's <stdio.h> already defines stdin, stdout, stderr tmpfile() as macros/inline functions. Frida's shim redefines them, causing compile errors.