powerpc/pseries: hcall functions are exported to modules, need _GLOBAL_TOC()

The hcall macros may call out to c code for tracing, so we need
to set up a valid r2. This fixes an oops found when testing
ibmvscsi as a module.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>