riscv: s64ilp32: Introduce xlen_t for 64ILP32 kernel

When s64ilp32 landed, we couldn't use CONFIG_64/32BIT to distingue XLEN
data types. Because the xlen is 64, but the long & pointer is 32 for
s64ilp32, and s64ilp32 is a 32BIT from the software view. So introduce a
new data type - "xlen_t" and use __riscv_xlen instead of CONFIG_64/32BIT
ifdef macro.

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