[S390] Export store_status() function

For kdump we need a store status function to save the registers for the
current CPU. Therefore this patch exports a function "store_status()".
In addition to that now also floating point registers are saved correctly.

Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>