S/390: Use proper read-only data section for literals.

S/390: Use proper read-only data section for literals.

Previously, .rodata was hardcoded.  For C++ vague linkage functions,
this resulted in needlessly duplicated literals.  With the new split
stack support, this resulted in link errors, due to .rodata containing
relocations to the discarded text sections.

gcc/ChangeLog:

	* config/s390/s390.md (pool_section_start): Use switch_to_section
	to select proper read-only data section instead of hardcoding .rodata.
	(pool_section_end): Use switch_to_section to match the above.

From-SVN: r232667