runtime: mark go-context.S as no-executable-stack and split-stack supported

runtime: mark go-context.S as no-executable-stack and split-stack supported

    The .note.GNU-stack section tells the linker that this object does not
    require an executable stack.

    The .note.GNU-split-stack section tells the linker that functions in
    this object can be called directly by split-stack functions, without
    require a large stack.

    The .note.GNU-no-split-stack section tells the linker that functions
    in this object do not have a split-stack prologue.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/198440

From-SVN: r276488