libstdc++: Add "cold" to tests for reserved attribute names

libstdc++: Add "cold" to tests for reserved attribute names

libstdc++-v3/ChangeLog:

	* testsuite/17_intro/headers/c++1998/all_attributes.cc: Check
	"cold" isn't used in the library. Also check <cxxabi.h>.
	* testsuite/17_intro/headers/c++2011/all_attributes.cc:
	Likewise.
	* testsuite/17_intro/headers/c++2014/all_attributes.cc:
	Likewise.
	* testsuite/17_intro/headers/c++2017/all_attributes.cc:
	Likewise.
	* testsuite/17_intro/headers/c++2020/all_attributes.cc:
	Likewise.