Don't add DT_INIT/DT_FINI for undef and shared symbols.

The freebsd dynamic linker doesn't check if the value is null (and it
is reasonable for it to do that). That means that producing a .so with
a null DT_INIT/DT_FINI causes the base address to be called.

This should fix the libreoffice build.

llvm-svn: 291944