s390: compile head.S always with -march=z900

head.S on s390 contains some sanity checks if the kernel will run on a
machine or if the machine is too old, e.g. if the kernel contains
instructions not available on the machine. If so, it will emit an error
message to the console before it stops execution.

Therefore head.S contains only instructions which are availanble with the
earliest machine generation (z900).  In order to make sure we don't
accidently add instructions which are not available on z900, always compile
with -march=z900. This makes sure compilation will fail if wrong
instructions are used.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>