powerpc: Don't use local named register variable in current_thread_info

LLVM doesn't support local named register variables and is unlikely
to. current_thread_info is using one, fix it by moving it out and
calling it __current_r1().

I gave it a bit of an obscure name because we don't want anyone else
using it - they should use current_stack_pointer(). This specific
case is performance critical and we can't afford to call a function
to get it. Furthermore it isn't important to know exactly where in
the stack we are since we mask the lower bits.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>