steam-runtime-abi: compare libdrm.so.2 by symbols
The default comparison by name is usually not enough for libdrm.so.2 because it is stuck at version libdrm.so.2.4.0 since 2008.
Instead using the symbols comparison should give us better results. (T24106)
This patch is necessary for both Soldier and Sniper.
We can also apply it for Scout for completeness, even if it is so old that we never go wrong if we always choose the host version (the current behaviour).
/cc @smcv