ada-lex.l: Ignore register diagnostic also for g++ defaulting to ISO C++17

ada-lex.l: Ignore register diagnostic also for g++ defaulting to ISO C++17

Building with a really old flex and a really new g++ is probably not
recommended, but it should not cause compile errors.

gdb/ChangeLog:

	* ada-lex.l: Extend register warnings diagnostics comment for g++.

include/ChangeLog:

	* diagnostics.h (DIAGNOSTIC_IGNORE_DEPRECATED_REGISTER): Also define
	for GCC version 7.0 or higher.