LoongArch: Always select HAVE_VIRT_CPU_ACCOUNTING_GEN

Option HAVE_VIRT_CPU_ACCOUNTING_GEN is selected by default for 64bit
system in kconfig file arch/Kconfig. There is another selection in file
arch/loongarch/Kconfig if SMP is not selected. Indeed this option is
SMP-safe so it brings out some misunderstanding for non-SMP case. Here
always select option HAVE_VIRT_CPU_ACCOUNTING_GEN for future possible
32bit system (it is also 32bit-safe because we no longer use cputime_t).

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>