st/egl: Move module loading code to targets.

st/egl: Move module loading code to targets.

Several changes are made.  libegl.a no longer defines _eglMain.  It
defines functions to create and destroy a _EGLDriver instead.  The
creation function is called by the targets.  It takes an egl_g3d_loader
as its argument.  The loader is defined by the targets and is in charge
of creating st_api and pipe_screen.  This allows us to move the module
loading code to targets.  Lastly, the modules are now loaded as the
respective contexts are created.