You can use --print native-static-libs to show all libraries you need to link against. For example:
$ echo "" | rustc --print native-static-libs --crate-type staticlib -
note: Link against the following native artifacts when linking against this static library. The order and any duplication can be significant on some platforms.
note: native-static-libs: -ldl -lrt -lpthread -lgcc_s -lc -lm -lrt -lpthread -lutil -ldl -lutil