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>