selftests: kvm: make syncregs more reliable on s390

similar to commit 2c57da356800 ("selftests: kvm: fix sync_regs_test with
newer gccs") and commit 204c91ef ("KVM: selftests: do not blindly
clobber registers in guest asm") we better do not rely on gcc leaving
r11 untouched.  We can write the simple ucall inline and have the guest
code completely as small assembler function.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>