runtime: move osinit to Go

runtime: move osinit to Go

    This is a step toward updating libgo to 1.13.  This adds the 1.13
    version of the osinit function to Go code, and removes the
    corresponding code from the C runtime.  This should simplify future updates.
    Some additional 1.13 code was brought in to simplify this change.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/191717

From-SVN: r275010