compiler, runtime: copy slice code from Go 1.7 runtime

    
    Change the compiler handle append as the gc compiler does: call a
    function to grow the slice, but otherwise assign the new elements
    directly to the final slice.
    
    For the current gccgo memory allocator the slice code has to call
    runtime_newarray, not mallocgc directly, so that the allocator sets the
    TypeInfo_Array bit in the type pointer.
    
    Rename the static function cnew to runtime_docnew, so that the stack
    trace ignores it when ignoring runtime functions.  This was needed to
    fix the runtime/pprof tests on 386.
    
    Reviewed-on: https://go-review.googlesource.com/32218

From-SVN: r241667