[m-dev.] Deprecation of the jump grades

[M McDonough]

I read in a recent commit that the jump grades have been deprecated.

I just wanted to note that on systems outside of x86/amd64 (such as on
Armv6 and Sparc64) these are the only low-level C grades that work on
BSD systems other than `none` (as tested in NetBSD 8 on an rPi B and
OpenBSD 6.4 on a Sun UltraSparc III machine).

I am well aware there are many issues with the jump grades though, and
many grade combinations including jump don't compile, even though the
same combo would work with `none`.

It's not a huge problem for me, I mostly only use the low-level C
grades for the debugger and not for actual production use. I just
wanted to be sure this was known.