Fix symbol versions with shared libs

Fix symbol versions with shared libs

ELF files that refer to shared libs containing sym-versions, but
don't refer to any dynamic symbols with symbol versions (should happen
only with very simple shared libs) would generate an empty .gnu.version_r
section.  Some dynamic linker contain bugs in that they don't check
the section size or DT_VERNEEDNUM (which are both zero for such files
we generate) before accessing the first entry, and then bail out with
a message like

./a.exe: error while loading shared libraries: ./a1.so: unsupported
 version 25960 of Verneed record

(where the "version number" actually comes from neighboring bytes
from different sections).

So, there's not much choice, we simply must not generate such section.