LoongArch: Select ARCH_USE_MEMTEST

As of commit dce44566 ("mm/memtest: add ARCH_USE_MEMTEST"),
architectures must select ARCH_USE_MEMTESET to enable CONFIG_MEMTEST.

Commit 628c3bb4 ("LoongArch: Add boot and setup routines") added
support for early_memtest but did not select ARCH_USE_MEMTESET.

Fixes: 628c3bb4 ("LoongArch: Add boot and setup routines")
Tested-by: Erpeng Xu <xuerpeng@uniontech.com>
Tested-by: Yuli Wang <wangyuli@uniontech.com>
Signed-off-by: Yuli Wang <wangyuli@uniontech.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>