KVM: selftests: Compile s390 tests with -march=z10

KVM: selftests: Compile s390 tests with -march=z10

The guest used in s390 kvm selftests is not be set up to handle all
instructions the compiler might emit, i.e. vector instructions, leading
to crashes.
Limit what the compiler emits to the oldest machine model currently
supported by Linux.

Signed-off-by: Nina Schoetterl-Glausch <nsg@linux.ibm.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Link: https://lore.kernel.org/r/20230127174552.3370169-1-nsg@linux.ibm.com
Message-Id: <20230127174552.3370169-1-nsg@linux.ibm.com>
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>