arch/*: Update Kconfig symbol usage

- Update all symbols to use IS_ENABLED()
- Update non-romcc usage to use 'if' instead of '#if' where it
makes sense.

Change-Id: I5a84414d2d1631e35ac91efb67a0d4c1f673bf85
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/20005
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

src/arch/riscv/include/arch/cpu.h

@@ -31,7 +31,7 @@ struct thread;
 struct cpu_info {
 	device_t cpu;
 	unsigned long index;
-#if CONFIG_COOP_MULTITASKING
+#if IS_ENABLED(CONFIG_COOP_MULTITASKING)
 	struct thread *thread;
 #endif
 };