A hang in Chromium on MacOS seems related to attempting to load
libGLES_CM repeatedly, since it doesn't exist. While dlopen()
probably shouldn't be this slow, it could also be related to a
sandboxing issue. In any case, this cl attempts to go around the
issue by never trying to load a library twice.

Bug chromium:904346

Change-Id: I65122f0fc9acb4f8db2a606437c61796464f72ad
Reviewed-on: https://swiftshader-review.googlesource.com/c/22468Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>