bionic/tests/sys_ptrace_test.cpp: add an instruction that writes more than 64 bits

[enh-google]

we need to grab a V instruction to finish the missing piece of this test:

static void watchpoint_imprecise_child(Uint128_t& data) {
  raise(SIGSTOP);  // Synchronize with the tracer, let it set the watchpoint.

#if defined(__i386__) || defined(__x86_64__)
  asm volatile("movdqa %%xmm0, %0" : : "m"(data));
#elif defined(__arm__)
  asm volatile("stm %0, { r0, r1, r2, r3 }" : : "r"(&data));
#elif defined(__aarch64__)
  asm volatile("stp x0, x1, %0" : : "m"(data));
#elif defined(__riscv)
  UNUSED(data);
  GTEST_LOG_(INFO) << "missing riscv64 instruction to store > 64 bits in one instruction";
#endif
}

[enh-google]

        vsetivli        zero, 16, e8, m1, ta, ma
        vmv.v.i v8, 0
        vse8.v  v8, (a0)

might be what we want? @negge