cxl: fix build for GCC 4.6.x

GCC 4.6.3 does not support -Wno-unused-const-variable. Instead, use the
kbuild infrastructure that checks if this options exists.

Fixes: 2cd55c68 ("cxl: Fix build failure due to -Wunused-variable behaviour change")
Suggested-by: Michal Marek <mmarek@suse.com>
Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>