runtime, reflect: rewrite Go to FFI type conversion in Go

runtime, reflect: rewrite Go to FFI type conversion in Go

    As we move toward the Go 1.7 garbage collector, it's essential that all
    allocation of values that can contain Go pointers be done using the
    correct type descriptor.  That is simplest if we do all such allocation
    in Go code.  This rewrites the code that converts from a Go type to a
    libffi CIF into Go.

    Reviewed-on: https://go-review.googlesource.com/33353

From-SVN: r242578