KVM: selftests: Limit dirty_log_test's s390x workaround to s390x

s390 specific workaround causes the dirty-log mode of the test to dirty
all guest memory on the first iteration, which is very slow when the test
is run in a nested VM.

Limit this workaround to s390x.

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Link: https://lore.kernel.org/r/20250111003004.1235645-9-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: Maxim Levitsky

tools/testing/selftests/kvm/dirty_log_test.c

@@ -98,6 +98,7 @@ static void guest_code(void)
 	uint64_t addr;
 	int i;
 
+#ifdef __s390x__
 	/*
 	 * On s390x, all pages of a 1M segment are initially marked as dirty
 	 * when a page of the segment is written to for the very first time.
@@ -108,6 +109,7 @@ static void guest_code(void)
 		addr = guest_test_virt_mem + i * guest_page_size;
 		vcpu_arch_put_guest(*(uint64_t *)addr, READ_ONCE(iteration));
 	}
+#endif
 
 	while (true) {
 		for (i = 0; i < TEST_PAGES_PER_LOOP; i++) {