s390x: initial infrastructure

This adds the very basic infrastructure for s390x. As we compile for
z900, also QEMU tcg can be used.

To cross compile:

$./configure --arch=s390x --cross-prefix=/usr/bin/s390x-linux-gnu-
$ make

Please note that for now, nothing will be compiled, as there is no test
to compile. A basic self test will be added in the following patches.

smp and stack unwinding is not supported yet. sclp console output will
be added in the following patches.

Parts based on a prototype by Thomas Huth.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20170522085702.17856-2-david@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>