Add support for LinuxThreads.

When using LinuxThreads pthread_setspecific triggers recursive
allocation on all threads. Work around this by creating a global linked
list of in-progress tsd initializations.

This modifies the _tsd_get_wrapper macro-generated function. When it has
to initialize an TSD object it will push the item to the linked list
first. If this causes a recursive allocation then the _get_wrapper
request is satisfied from the list. When pthread_setspecific returns the
item is removed from the list.

This effectively adds a very poor substitute for real TLS used only
during pthread_setspecific allocation recursion.

Signed-off-by: Crestez Dan Leonard <lcrestez@ixiacom.com>