[arch] have each arch define ARCH_HAS_MMU

This lets some code decide whether or not there's any mmu
present to use. Also kernel VM will complain if it isn't set
as an extra safety.

arch/riscv/arch.c

@@ -59,7 +59,6 @@ void arch_early_init(void) {
 
 #if RISCV_S_MODE
     sbi_early_init();
-
 #endif
 #if RISCV_MMU
     riscv_early_mmu_init();