notes.texi: Specify -lasan with -fsanitize=address in g++

notes.texi: Specify -lasan with -fsanitize=address in g++

When using the flag -fsanitize=address with g++, the user should also
specify -lasan to ensure linking occurs correctly. libtool uses
-nostdlib, and gcc implicitly includes -lasan when -fsanitize=address
is used. However, g++ does not implicitly include -lasan. The
documentation has been updated to reflect this information rather than
adding the -lasan flag to libtool.

* doc/notes.texi: Document that the user should pass -lasan.