s390: Document s390 stackprotector support

Recently [1] s390 got stackprotector support. Document this.

[1] commit f5730d4 ("s390: Add stackprotector support")

Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

Author: hcahca

Documentation/features/debug/stackprotector/arch-support.txt

@@ -21,7 +21,7 @@
     |      parisc: | TODO |
     |     powerpc: |  ok  |
     |       riscv: |  ok  |
-    |        s390: | TODO |
+    |        s390: |  ok  |
     |          sh: |  ok  |
     |       sparc: | TODO |
     |          um: | TODO |