[S390] compile fix for latest binutils

The latest binutils won't accept the stfl instruction with march=g5
which is the correct behaviour. Unfortunately head.S is assembled
with -march=g5 even if the target cpu is z900 or later. To get
31-bit kernels compiled again the easiest fix is to use the .insn
notation for the stfl instruction in head.S.

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