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>