Allow an info.size field of -1 to indicate that the descriptor should map

Allow an info.size field of -1 to indicate that the descriptor should map
the entire address space, requires for direct-tls %gs:OFFSET support (where
the OFFSET is negative).  Otherwise the descriptor only maps the specified
range (typically just the TCB area created by the thread library) and %gs:0
must be used to obtain a pointer from which negative relative memory accesses
can be made.