s390: Always compute address of stack protector guardHEADtrunkmaster

Computing the address of the thread pointer on s390 involves multiple
instructions and therefore bears the risk that the address of the canary
or intermediate values of it are spilled after prologue in order to be
reloaded for the epilogue.  Since there exists no mechanism to ensure
that a value is not coming from stack, as a precaution compute the
address always twice, i.e., one time for the prologue and one time for
the epilogue.  Note, even if there were such a mechanism, emitting
optimal code is non-trivial since there exist cases with opposing
requirements as e.g. if the thread pointer is not only computed for the
TLS guard but also for other TLS objects.  For the latter accesses it is
desired to spill and reload the thread pointer instead of recomputing it
whereas for the former it is not.

gcc/ChangeLog:

	* config/s390/s390.md (stack_protect_get_tpsi): New insn.
	(stack_protect_get_tpdi): New insn.
	(stack_protect_set): Use new insn.
	(stack_protect_test): Use new insn.

gcc/testsuite/ChangeLog:

	* gcc.target/s390/stack-protector-guard-tls-1.c: New test.

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