vmlinux.lds.h: replace config_enabled() with IS_ENABLED()

The use of config_enabled() against config options is ambiguous.

Now, IS_ENABLED() is implemented purely with macro expansion, so
let's replace config_enabled() with IS_ENABLED().

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Michal Marek <mmarek@suse.com>