When doing a runtimes build with LTO using ld.bfd (or ld.gold), the
build starts failing with ninja 1.12, which added a new critical path
scheduler. The reason is that LLVMgold.so is not available yet at the
point where runtimes start being build, leading to configuration
failures in the nested cmake invocation.
Fix this by adding an explicit dependency on LLVMgold.so if it is
available. (It may not always be necessary, e.g. if the used linker is
lld, but it would be hard to detect when exactly it may or may not be
needed, so always adding the dependency is safer.)
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