riscv: s64ilp32: Add ARCH_RV64ILP32 Kconfig option

Just the same as ARCH_RV64I & ARCH_RV32I, add ARCH_RV64ILP32 config
for s64ilp32 and turn on the s64ilp32 compile switch in the
arch/riscv/Makefile.

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