swapcontext rewrite broke some software

[Oliver Pinter]

Hi!

After this commit:

commit ac0cfc7fcb1b51ee6aeacfd676fa6dfbe11eefb5
Author: davidxu <davidxu at FreeBSD.org>
Date:   Wed Apr 10 02:40:03 2013 +0000

    swapcontext wrapper can not be implemented in C, the stack pointer saved in
    the context becomes invalid when the function returns, same as setjmp,
    it must be implemented in assemble language, see discussions in PR
    misc/177624.

Some* software not found the swapcontext functions after this commit.
Please add a sentence to UPDATING file and/or bump the
__FreeBSD_version to reflect this change.


* qemu