Remove absolute path from libust.so

Remove absolute path from libust.so

I noticed that the libust.so ld script is installed to the same directory as
libust.so.0 and libust-initializer.o, so removing the absolute paths from them
allows linking with a libust.so that isn't in the sysroot or installed under
/usr/, like e.g., in a DESTDIR directory.  This was useful for testing a
release with everything extracted some randome prefix, e.g., libust.so in
$RANDOM_UNPACK_DIR/usr/lib, and pointing LD_LIBRARY_PATH to
$RANDOM_UNPACK_DIR/usr/lib.