s390: add support for IBM z14 machines

Add detection for machine type 0x3906 and set the ELF platform name
to z14. Add the miscellaneous-instruction-extension 2 facility to
the list of facilities for z14.

And allow to generate code that only runs on a z14 machine.

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