Documentation/stackprotector: powerpc supports stack protector

powerpc architecture (both 64-bit and 32-bit) supports stack protector
mechanism since some time now [see commit 06ec27ae ("powerpc/64:
add stack protector support")].

Update stackprotector arch support documentation to reflect the same.

Cc: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Bhupesh Sharma <bhsharma@redhat.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

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

@@ -22,7 +22,7 @@
     |       nios2: | TODO |
     |    openrisc: | TODO |
     |      parisc: | TODO |
-    |     powerpc: | TODO |
+    |     powerpc: |  ok  |
     |       riscv: | TODO |
     |        s390: | TODO |
     |          sh: |  ok  |