GL: drop symbols mangling support

SCons and Meson have never supported that feature, and Autotools was
deleted over 6 months ago and no-one complained yet, so it's pretty
obvious nobody cares about it.

Fixes: 95aefc94 ("Delete autotools")
Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Acked-by: Dylan Baker <dylan@pnwbakers.com>
(cherry picked from commit a0829cf2)