GDC: druntime GC wrongly frees data pointed to by TLS

[dsimcha]

Bug 2491 (http://d.puremagic.com/issues/show_bug.cgi?id=2491) was resolved a
long time ago in DMD, but now a similar issue is occurring in GDC.
(http://bitbucket.org/goshawk/gdc/issue/115/weird-memory-corruption-error-w-tls-gc)
 Can whoever fixed 2491 (I think it was Walter) please provide information
about how the DMD version of this bug was fixed, or any other leads that might
help it get fixed in GDC?