[S390] Kconfig: add machine type number to code generation options

Add machine type number to code generation options. Also clean up and
shorten quite a lot of help texts with respect to machine type and
architecture terminology.

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