Remove <unordered_set> usages from abg_compat

Remove <unordered_set> usages from abg_compat

std::unordered_set is now provided through the language standard, hence
remove the compatibility code for <unordered_set> and adjust all users
accordingly.

	* include/abg-cxx-compat.h: Drop compatibility for <unordered_set>.
	* include/abg-comparison.h: migrate abg_compat use to std.
	* include/abg-interned-str.h: Likewise.
	* include/abg-suppression.h: Likewise.
	* src/abg-comparison-priv.h: Likewise.
	* src/abg-dwarf-reader.cc: Likewise.
	* tests/test-diff-suppr.cc: Likewise.
	* tools/abipkgdiff.cc: Likewise.

Signed-off-by: Matthias Maennich <maennich@google.com>