libstdc++: Update list of known symbol versions for abi-check

Add the recently added CXXABI_1.3.15 version. Also remove two "frozen"
versions from the latestp list, as no more symbols should be added to
those now.

libstdc++-v3/ChangeLog:

	* testsuite/util/testsuite_abi.cc (check_version): Add
	CXXABI_1.3.15 symver and make it the latestp. Remove
	GLIBCXX_IEEE128_3.4.31 and GLIBCXX_LDBL_3.4.31 from latestp.