[PATCH gold/21066] Consider C++ exception handling metadata during ICF

[Joshua Oreman]

Two functions with the same code might not be combinable during ICF if
they have different C++ exception handling metadata. PR gold/21066
demonstrates this for the case where only the LSDA differs. New
optimizations in GCC 8 can also create cases where the code is
identical but the unwind tables differ; see gold/23482 for more
details. This patch extends ICF to consider a function's associated
FDE and CIE, including any relocations therein (e.g. to the LSDA),
when deciding whether two functions are identical or not.

2018-09-24  Joshua Oreman  <oremanj@hudson-trading.com>

       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.
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