riscv: s64ilp32: Add rv64ilp32_defconfig

Follow the rv32_defconfig rule to add rv64ilp32_defconfig; the only
difference is:
-CONFIG_ARCH_RV32I=y
+CONFIG_ARCH_RV64ILP32=y

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Guo Ren <guoren@kernel.org>