Fix linkage1 test which fails due to --as-needed

It appears that the --as-needed flag to linkers will not pull in a dynamic library unless it satisfies a non weak undefined symbol. The linkage1 test was creating a dynamic library where it was only used for a weak-symbol as part of an executable, so the dynamic library was getting discarded.

This commit adds another symbol to the library which satisfies a strong undefined symbol, so the library is pulled in to resolve the weak reference.
auto merge of #12983 : nick29581/rust/test_fix, r=alexcrichton