riscv: Add support for BATCHED_UNMAP_TLB_FLUSH

Allow to defer the flushing of the TLB when unmapping pages, which allows
to reduce the numbers of IPI and the number of sfence.vma.

The ubenchmarch used in commit 43b3dfdd ("arm64: support
batched/deferred tlb shootdown during page reclamation/migration") that
was multithreaded to force the usage of IPI shows good performance
improvement on all platforms:

* Unmatched: ~34%
* TH1520   : ~78%
* Qemu     : ~81%

In addition, perf on qemu reports an important decrease in time spent
dealing with IPIs:

Before:  68.17%  main     [kernel.kallsyms]            [k] __sbi_rfence_v02_call
After :   8.64%  main     [kernel.kallsyms]            [k] __sbi_rfence_v02_call

* Benchmark:

int stick_this_thread_to_core(int core_id) {
        int num_cores = sysconf(_SC_NPROCESSORS_ONLN);
        if (core_id < 0 || core_id >= num_cores)
           return EINVAL;

        cpu_set_t cpuset;
        CPU_ZERO(&cpuset);
        CPU_SET(core_id, &cpuset);

        pthread_t cur...