s390/boot: Increase minimum architecture to z10

s390/boot: Increase minimum architecture to z10

The decompressor code is partially compiled with march=z900 so it is
possible to print an error message in case a kernel is booted on a
machine which misses facilities to execute the kernel.

Given that the decompressor code also includes header files from the
core kernel this causes problems for inline assemblies and other code
where the minimum assumed architecture level is set to z10 in the
meantime. If such code is also used in the decompressor (e.g. inline
functions) z900 support must be implemented again.

In order to avoid this and to keep things simple just raise the
minimum architecture level to z10 for the decompressor just like for
the kernel.

Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>