Compiler Attributes: fix comment concerning GCC 4.6

GCC 4.6 is not supported anymore, so remove a reference to it,
leaving just the part about version prior GCC 5.

Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Signed-off-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>