s390/dump: remove SAVE_AREA_BASE

Replace the SAVE_AREA_BASE offset calculations in reipl.S with the
assembler constant for the location of each register status area.

Use __LC_FPREGS_SAVE_AREA instead of SAVE_AREA_BASE in the three
remaining code locations and remove the definition of SAVE_AREA_BASE.

Acked-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>