selftests/vDSO: Add riscv getcpu & gettimeofday test

Enable vDSO getcpu & gettimeofday test for riscv. But only riscv64
supports __vdso_gettimeofday and riscv32 is under development.

VERSION
{
        LINUX_4.15 {
        global:
                __vdso_rt_sigreturn;
                __vdso_gettimeofday;
                __vdso_clock_gettime;
                __vdso_clock_getres;
                __vdso_getcpu;
                __vdso_flush_icache;
        local: *;
        };
}

Co-developed-by: haocheng.zy <haocheng.zy@linux.alibaba.com>
Signed-off-by: haocheng.zy <haocheng.zy@linux.alibaba.com>
Suggested-by: Mao Han <han_mao@linux.alibaba.com>
Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Guo Ren <guoren@kernel.org>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Elliott Hughes <enh@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>