package/gcc: drop negative conditions on C-SKY for older versions

Now that the C-SKY architecture requires gcc-9, we can drop the special
conditions on the individual older versions.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Guo Ren <guoren@kernel.org>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Romain Naour <romain.naour@gmail.com>
Acked-by: Guo Ren <guoren@kernel.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>