compiler: drop special handling of unexported func/var assembler names

compiler: drop special handling of unexported func/var assembler names

    For example, for the package math/big, we used to generate unexported
    names as `big.trim` and exported names as `math_big.NewInt`.  After
    this change we will use `math_big` consistently.

    Reviewed-on: https://go-review.googlesource.com/68651

From-SVN: r253468