s390: Support global stack protector

s390: Support global stack protector

So far only a per thread canary in the TLS block is supported.  This
patch adds support for a global canary, too.  For this the new option
-mstack-protector-guard={global,tls} is added which defaults to tls.

The global canary is expected at symbol __stack_chk_guard which means
for a function prologue instructions larl/l(g)fr + mvc are emitted and
for an epilogue larl/l(g)fr + clc.

Furthermore, option -mstack-protector-guard-record is added which is
inspired by -mrecord-mcount and generates section __stack_protector_loc
containing pointers to all instructions which load the address of the
global guard.  Thus, this option has only an effect in conjunction with
-mstack-protector-guard=global.  The intended use is for the Linux
kernel in order to support run-time patching.  In each task_struct of
the kernel a canary is held which will be copied into the lowcore.
Since the kernel supports migration of the lowcore at boot time,
addresses are not necessarily constant.  Therefore, the kernel expects
that all instructions loading the address of the canary to be of format
RIL or more precisely are either larl or lgrl and that the instructions
addresses are recorded in section __stack_protector_loc.  The kernel is
then required to patch those instructions e.g. to llilf during boot.

In total this means -mstack-protector-guard=global emits code suitable
for user and kernel space.

gcc/ChangeLog:

	* config/s390/s390-opts.h (enum stack_protector_guard): Define
	SP_TLS and SP_GLOBAL.
	* config/s390/s390.h (TARGET_SP_GLOBAL_GUARD): Define predicate.
	(TARGET_SP_TLS_GUARD): Define predicate.
	* config/s390/s390.md (stack_protect_global_guard_addr<mode>):
	New insn.
	(stack_protect_set): Also deal with a global guard.
	(stack_protect_test): Also deal with a global guard.
	* config/s390/s390.opt (-mstack-protector-guard={global,tls}):
	New option.
	(-mstack-protector-guard-record) New option.

gcc/testsuite/ChangeLog:

	* gcc.target/s390/stack-protector-guard-global-1.c: New test.
	* gcc.target/s390/stack-protector-guard-global-2.c: New test.
	* gcc.target/s390/stack-protector-guard-global-3.c: New test.
	* gcc.target/s390/stack-protector-guard-global-4.c: New test.