compiler: don't export function descriptors for unexported names

compiler: don't export function descriptors for unexported names

    They aren't needed, and could potentially cause unlikely symbol name
    collisions.  Also, the runtime package's reference to main could cause
    the runtime package to define main.main..f, which could also be
    defined in the main package if it does something like fmt.Print(main).
    That will normally work but will fail with a multiple symbol
    definition error when using -static-libgo.

    Reviewed-on: https://go-review.googlesource.com/93656

From-SVN: r257637