Fix OPENBLAS_OBJ_TARGET on Linux.

Currently symlink to libopenblas.so.0 from libopenblas.so contains full path, which is not correct. So I had following case
```
[baur@linux julia-89cafb9b8a]$ ll lib/julia/libopenblas*
-rwxr-xr-x 1 baur baur 32582112 Sep 15 12:08 lib/julia/libopenblas.so
lrwxrwxrwx 1 baur baur       55 Sep 15 15:32 lib/julia/libopenblas.so.0 -> /builddir/build/BUILD/julia-test/usr/lib/libopenblas.so
```
You can see that libopenblas.so.0 was symlinked with incorrect path.