[smart-select:] Use a linker version script, to remove even more deadcode.

The size of our 32-bit lib went down 264K -> 180K (1K = 1024 bytes).

$ mmma -j32 external/libtextclassifier
$ ls -sh $OUT/system/lib/libtextclassifier.so
180K <SKIPPED/LONG/PATH/TO>/system/lib/libtextclassifier.so
$ # :)

Treemap info:
https://cnsviewer.corp.google.com/cns/ig-d/home/salcianu/libtextclassifier/treemap/android_03_24.3/index.html

The gap between the treemap measured code size and the .so file size is now down
to 40K, i.e., 22% (down from 43% this morning).

Previously, we were using only -fvisibility=hidden for our code.  That impacted
only the symbols exported from our code, not the symbols from the static libs we
depend on.  The linker script handles everything.

Test: manual test that everything builds + inspection of treemap info.

All required code seems to be linked in.

Change-Id: I00bfea854a80ef365b229632d2af259c938159c2