clocksource: riscv: s64ilp32: Use __riscv_xlen instead of CONFIG_32BIT

When s64ilp32 enabled, CONFIG_32BIT=y but __riscv_xlen=64. So we
must use __riscv_xlen to detect real machine XLEN for CSR access.

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