s390: use -fPIC for module compile

The xfs module uses a lot of tracepoint, with TRACEPOINTS=y and a
few debugging options the GOT table of the xfs module will get
bigger than 4K. To get a working xfs module it needs to be compiled
with -fPIC instead of -fpic. To play safe use -fPIC for all modules.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>