linux_dl: Update stale comments before Mesa shipped ES3

Before Mesa shipped ES3, Waffle speculated that the library name would
remain libGLESv2.so. Speculation became reality. Update some comments
to replace that speculation with a statement of fact.

Reported-by: Jordan Justen <jordan.l.justen@intel.com>
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>