[Sanitizers] Call NanoTime() conditionally.

[alekseyshl]

Summary

Call NanoTime() in primary 64 bit allocator only when necessary,
otherwise the unwarranted syscall causes problems in sandbox environments.
ReleaseToOSIntervalMs() conditional allows them to turn the feature off
with allocator_release_to_os_interval_ms=-1 flag.

compiler-rt/trunk/lib/sanitizer_common/sanitizer_allocator_primary64.h

if (new_space_end > region->mapped_user) {
       if (UNLIKELY(region->mapped_user == 0)) {
         if (!kUsingConstantSpaceBeg && kRandomShuffleChunks)
           // The random state is initialized from ASLR.
           region->rand_state = static_cast<u32>(region_beg >> 12);
         // Postpone the first release to OS attempt for ReleaseToOSIntervalMs,
         // preventing just allocated memory from being released sooner than
         // necessary and also preventing extraneous ReleaseMemoryPagesToOS calls
         // for short lived processes.
+        // Do it only when the feature is turned on, to avoid a potentially
+        // extraneous syscall.
+        if (ReleaseToOSIntervalMs() >= 0)
           region->rtoi.last_release_at_ns = NanoTime();
       }
       // Do the mmap for the user memory.
       uptr map_size = kUserMapSize;
       while (new_space_end > region->mapped_user + map_size)
         map_size += kUserMapSize;
       CHECK_GE(region->mapped_user + map_size, new_space_end);
       if (UNLIKELY(!MapWithCallback(region_beg + region->mapped_user,
                                     map_size)))