Relocatable read-only section support for absolute jump table

Relocatable read-only section support for absolute jump table

This patch puts absolute jump tables into a relocatable read-only section
if they are on ELF target and relocation is supported.

gcc/ChangeLog:

	* final.c (final_scan_insn_1): Set jump table relocatable as the
	second argument of targetm.asm_out.function_rodata_section.
	* output.h (default_function_rodata_section,
	default_no_function_rodata_section): Add the second argument to the
	declarations.
	* target.def (function_rodata_section): Change the doc and add
	the second argument.
	* doc/tm.texi: Regenerate.
	* varasm.c (jumptable_relocatable): Implement.
	(default_function_rodata_section): Add the second argument
	and the support for relocatable read only sections.
	(default_no_function_rodata_section): Add the second argument.
	(function_mergeable_rodata_prefix): Set the second argument to false.
	* config/mips/mips.c (mips_function_rodata_section): Add the second
	arugment and set it to false.
	* config/s390/s390.c (targetm.asm_out.function_rodata_section): Set
	the second argument to false.
	* config/s390/s390.md: Likewise.