`isprecompile(pkg) == false` for stdlibs in base sysimage

MWE:
```
julia> pkg = Base.identify_package("LinearAlgebra")
LinearAlgebra [37e2e46d-f89d-539d-b4ee-838fcccc9c8e]

julia> Base.isprecompiled(pkg)
false
```

Whereas `Statistics` was already moved out, so its fine:
```
julia> pkg = Base.identify_package("Statistics")
Statistics [10745b16-79ce-11e8-11f9-7d13ad32a3b2]

julia> Base.isprecompiled(pkg)
true
```