compiler: add go:notinheap magic comment

compiler: add go:notinheap magic comment

    Implement go:notinheap as the gc compiler does. A type marked as
    go:notinheap may not live in the heap, and does not require a write
    barrier. Struct and array types that incorporate notinheap types are
    themselves notinheap. Allocating a value of a notinheap type on the
    heap is an error.

    This is not just an optimization. There is code where a write barrier
    may not occur that was getting a write barrier with gccgo but not gc,
    because the types in question were notinheap. The case I found was
    setting the mcache field in exitsyscallfast.

    Reviewed-on: https://go-review.googlesource.com/46490

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@249594 138bc75d-0d04-0410-961f-82ee72b054a4