runtime: copy channel code from Go 1.7 runtime

runtime: copy channel code from Go 1.7 runtime

    Change the compiler to use the new routines. Drop the separation of
    small and large values when sending on a channel. Allocate the select
    struct on the stack. Remove the old C implementation of channels. Adjust
    the garbage collector for the new data structure.

    Bring in part of the tracing code, enough for the channel code to call.

    Bump the permitted number of allocations in one of the tests in
    context_test.go. The difference is that now receiving from a channel
    allocates a sudog, which the C code used to simply put on the
    stack. This will be somewhat better when we port proc.go.

    Reviewed-on: https://go-review.googlesource.com/30714

From-SVN: r240941