1, ACPI, EFI and SMP are mandatories for LoongArch, select them
unconditionally to avoid various build errors for 'make randconfig'.
2, Move the MMU_GATHER_MERGE_VMAS selection to the correct place.
Reported-by: Nkernel test robot <lkp@intel.com>
Signed-off-by: NHuacai Chen <chenhuacai@loongson.cn>