riscv: 32-bit adding __ARCH_WANT_TIME32_SYSCALLS for syscalls

[Yimin Gu <ustcymgu@xxxxxxxxx>]

In include/uapi/asm-generic/unistd.h, syscalls guarded by the
__ARCH_WANT_TIME32_SYSCALLS macro, like adjtimex, are not present in
32-bit RISC-V kernels because the macro is not defined for riscv32.

Will it be good to add the __ARCH_WANT_TIME32_SYSCALLS, as well as
__ARCH_WANT_STAT64 and __ARCH_WANT_SET_GET_RLIMIT macros for riscv32
in arch/riscv/include/uapi/asm/unistd.h? I got uclibc/busybox compiled and
run on riscv32 nommu systems after adding these macros, but I'm not sure
if there's any potential problems.