Fix a nasty bug: when saving errno in the TSO after running a thread,
beware that the TSO might have moved, e.g. if the thread made a safe
foreign call in the threaded RTS and a GC happened.

We have to grab the new location of the TSO *before* saving errno.

MERGE TO STABLE

This was the most interesting bug hunt I've had in a while.  The crash
only showed up in about 1 in 4 runs of a program with 1000 Haskell
threads, running on the threaded RTS.  To make things worse, gdb
doesn't support watchpoints in programs with multiple threads... :-/