Use the correct command to get architecture

`uname -m` was used by mistake. As a result, kexec-tools failed to
update crashkernel=auto during in-place upgrade from RHEL8 to RHEL9.

`uname -m` should be used to get architecture instead.

Fixes: 5951b5e2 ("Don't try to update crashkernel when bootloader is not installed")

Signed-off-by: Coiby Xu <coxu@redhat.com>
Reviewed-by: Lichen Liu <lichliu@redhat.com>

kdumpctl

@@ -1525,7 +1525,7 @@ reset_crashkernel()
 
 _is_bootloader_installed()
 {
-	if [[ $(uname -r) == s390x ]]; then
+	if [[ $(uname -m) == s390x ]]; then
 		test -f /etc/zipl.conf
 	else
 		test -f /boot/grub2/grub.cfg