This file is based in both coroutine-ucontext.c and
pth_mctx.c (from the GNU Portable Threads library).

The mechanism used to change stacks is the sigaltstack
function (variant 2 of the pth library).

v2: Some corrections. Moving global variables into
thread storage (CoroutineThreadState).

Signed-off-by: Alex Barcelo <abarcelo@ac.upc.edu>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>