I have encountered the same problem.
Note that the file .../extensions/libglx.so has been there for quite
some time. What changed is that the OS name ("linux") is no longer used
for populating the subdirectories that are used to refine the module
path:
https://cgit.freedesktop.org/xorg/xserver/commit/hw/xfree86/loader/loadmod.c?id=97bd6e453676516891250389ec0fd695c110087c
A possible solution is to create the libglx.so symlink directly inside
/usr/lib/xorg/modules/ rather than in /usr/lib/xorg/modules/linux/.
Alternatively, you can divert the existing .../extensions/libglx.so
file. I'm not sure what the best solution is.
Best regards,
Bertram Felgenhauer
