[S390] correct address of _stext with CONFIG_SHARED_KERNEL=y

As of git commit 1844c9bc head64.S/head31.S
are not included in head.S anymore but build as an extra object. This breaks
shared kernel support because the .org statement in head64.S/head31.S for
CONFIG_SHARED_KERNEL=y will have a different effect. The end address of the
head.text section in head.o will be added to the .org value, to compensate
for this subtract 0x11000 to get the required value of 0x100000 again.

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