- Add new test "s390x_gs_threads" for GS with broadcast command for the
s390_guarded_storage() system call.

 - Add GS control blocks in test s390x_regs_check.c

The original test case for GS has been provided by
Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Alice Frosi <alice@linux.vnet.ibm.com>
Reviewed-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>