s390: Move -msoft-float/-fno-optimize-sibling-calls into compel Makefiles

We currently define "CFLAGS += -msoft-float -fno-optimize-sibling-calls"
in Makefile.compel.

Makefile.compel is included into the toplevel Makefile and so changes
CFLAGS which are exported to all criu and zdtm Makefiles.

We must not use -msoft-float outside the compel files. E.g. otherwise for
zdtm we get the following build error:

uptime_grow.o: In function `main':
/tmp/2/criu/test/zdtm/static/uptime_grow.c:36: undefined reference to `__floatdidf'
/tmp/2/criu/test/zdtm/static/uptime_grow.c:36: undefined reference to `__muldf3'
/tmp/2/criu/test/zdtm/static/uptime_grow.c:36: undefined reference to `__floatdidf'
/tmp/2/criu/test/zdtm/static/uptime_grow.c:36: undefined reference to `__adddf3

Fix this and move the CFLAGS definition to the compel Makefiles.
We do this by defining a new flag CFLAGS_PIE that is only used for pie code.

Reported-by: Adrian Reber <areber@redhat.com>
Suggested-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Reviewed-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Tested-by: Adrian Reber <areber@redhat.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>