LoongArch: Select THP_SWAP if HAVE_ARCH_TRANSPARENT_HUGEPAGE

THP_SWAP has been proven to improve the swap throughput significantly on
x86_64 system according to commit bd4c82c2 ("mm, THP, swap: delay
splitting THP after swapped out"), on ARM64 system according to commit
d0637c50 ("arm64: enable THP_SWAP for arm64") and on RISC-V system
according to commit 87f81e66 ("riscv: enable THP_SWAP for RV64").

Enable THP_SWAP for LoongArch, testing the micro-benchmark which is
introduced by commit d0637c50 ("arm64: enable THP_SWAP for arm64")
shows below numbers on the Loongson-3A5000 board:

swp out bandwidth w/o patch: 1815716 bytes/ms (mean of 10 tests)
swp out bandwidth w/  patch: 3410003 bytes/ms (mean of 10 tests)

Improved by 46.75%!
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

arch/loongarch/Kconfig

@@ -70,6 +70,7 @@ config LOONGARCH
 	select ARCH_WANT_LD_ORPHAN_WARN
 	select ARCH_WANT_OPTIMIZE_HUGETLB_VMEMMAP
 	select ARCH_WANTS_NO_INSTR
+	select ARCH_WANTS_THP_SWAP if HAVE_ARCH_TRANSPARENT_HUGEPAGE
 	select BUILDTIME_TABLE_SORT
 	select COMMON_CLK
 	select CPU_PM