Fixes Mac build errors from 2 recent commits:
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i#2990 7040ca7f added references to si_addr_lsb which are not on older Linux kernels or Mac. I put in our own core/unix/include/siginfo.h so we can have the latest definition. There is padding in the struct, so on Linux we can blindly write to new fields and not worry about the underlying kernel being old.
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i#2950 e1b6a66a added struct rlimit64 references which we typedef here to struct rlimit on Mac.
Issue: #2990 (closed), #2950 (closed)