[SystemZ] Support global stack protector canary

[stefan-sf-ibm]

So far only a per thread canary in the TLS block is supported on SystemZ. This patch for GCC adds support for a global canary, too. For this the new option -mstack-protector-guard={global,tls} is added which defaults to tls. As described in the patch, also option -mstack-protector-guard-record is added. More details are in the commit message.

\cc @uweigand @dominik-steenken