riscv: enable THP_SWAP for RV64

I have a Sipeed Lichee RV dock board which only has 512MB DDR, so
memory optimizations such as swap on zram are helpful. As is seen
in commit d0637c50 ("arm64: enable THP_SWAP for arm64") and
commit bd4c82c2 ("mm, THP, swap: delay splitting THP after
swapped out"), THP_SWAP can improve the swap throughput significantly.

Enable THP_SWAP for RV64, testing the micro-benchmark which is
introduced by commit d0637c50 ("arm64: enable THP_SWAP for arm64")
shows below numbers on the Lichee RV dock board:

swp out bandwidth w/o patch: 66908 bytes/ms (mean of 10 tests)
swp out bandwidth w/ patch: 322638 bytes/ms (mean of 10 tests)

Improved by 382%!
Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20220829145742.3139-1-jszhang@kernel.org/Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/Kconfig

@@ -45,6 +45,7 @@ config RISCV
 	select ARCH_WANT_FRAME_POINTERS
 	select ARCH_WANT_GENERAL_HUGETLB
 	select ARCH_WANT_HUGE_PMD_SHARE if 64BIT
+	select ARCH_WANTS_THP_SWAP if HAVE_ARCH_TRANSPARENT_HUGEPAGE
 	select BINFMT_FLAT_NO_DATA_START_OFFSET if !MMU
 	select BUILDTIME_TABLE_SORT if MMU
 	select CLONE_BACKWARDS