powerpc: Use global register variable in <thread_pointer.h>
A local register variable is merely a compiler hint, and so not
appropriate in this context. Move the global register variable into
<thread_pointer.h> and include it from <tls.h>, as there can only
be one global definition for one particular register.
Fixes commit
8dbeb0561eeb876f557ac9eef5721912ec074ea5
("nptl: Add <thread_pointer.h> for defining __thread_pointer").
Reported-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Reviewed-by: Raphael M Zinsly <rzinsly@linux.ibm.com>