s390: Introduce Start Arm Execution instruction

The Start Arm Execution (SAE) instruction is the centerpiece for
executing arm64 (KVM) guests on s390. Its purpose is, similar to SIE, to
enable accelerated execution of arm64 virtual machines. SAE expects the
physical address of a control block as the only argument.

The host is responsible to save & restore
 - GPRs 0-13
 - access registers 0-15
 - breaking event register (BEAR)
 - vector/floating point registers
between SAE executions to guarantee host consistency.

GPRs and BEAR are saved and restored in the asm functions. The other
registers are handled within C code. Access registers are handled in a
later patch and SVEs will be handled when they are introduced in a
future series. Most arm64 registers are handled by a satellite block
called save_area. Some registers, frequently used by hypervisors, are
placed into the SAE control block itself.

Enlighten asm/kvm_host_types.h for the new header variant. The new
header is chosen instead of asm/kvm_host_s390_types.h if KVM_S390_ARM64
is defined.

Co-developed-by: Andreas Grapentin <gra@linux.ibm.com>
Signed-off-by: Andreas Grapentin <gra@linux.ibm.com>
Co-developed-by: Nina Schoetterl-Glausch <nsg@linux.ibm.com>
Signed-off-by: Nina Schoetterl-Glausch <nsg@linux.ibm.com>
Reviewed-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Steffen Eiden <seiden@linux.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@linux.ibm.com>

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