Fix problem with ICF where diffs in EH frame info is ignored.

Fix problem with ICF where diffs in EH frame info is ignored.

	PR gold/21066
	* gc.h (gc_process_relocs): Track relocations in .eh_frame sections
	when ICF is enabled, even though the .eh_frame sections themselves
	are not foldable.
	* icf.cc (get_section_contents): Change arguments to permit operation
	on just part of a section. Include extra identity regions in the
	referring section's contents recursively.
	(match_sections): Lock object here instead of in get_section_contents
	so that get_section_contents can operate recursively.
	(Icf::add_ehframe_links): New method.
	(Icf::find_identical_sections): Pass .eh_frame sections to
	add_ehframe_links(). Increase default iteration count from 2 to 3
	because handling exception info typically requires one extra iteration.
	* icf.h (Icf::extra_identity_list_): New data member with accessor.
	(is_section_foldable_candidate): Include .gcc_except_table sections.
	* options.h: Update documentation for new default ICF iteration count.
	* testsuite/Makefile.am (icf_test_pr21066): New test case.
	* testsuite/Makefile.in: Regenerate.
	* testsuite/icf_test_pr21066.cc: New source file.
	* testsuite/icf_test_pr21066.sh: New test script.