There are some LSyms that are lazily initialized,
and which cannot be made eagerly initialized,
such as elements of a constant pool.
To avoid needing a mutex to protect the internals of
those LSyms, this CL introduces LookupInit,
which allows an LSym to be initialized only once.
By itself this is not fully concurrency-safe,
but Ctxt.Hash will need mutex protection anyway,
and that will be enough to support one-time LSym initialization.