[sanitizer] Reduce stack depot size on Android.

[eugenis]

Summary

The default setting kTabSizeLog=20 results in an 8Mb global hash table,
almost all of it in private pages. That is not a sane setting in a
mobile, system-wide use case: with ~150 concurrent processes stack
depot will account for more than 1Gb of RAM.

[kcc]

LGTM

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

struct StackDepotHandle {
   StackDepotHandle() : node_(nullptr) {}
   explicit StackDepotHandle(StackDepotNode *node) : node_(node) {}
   bool valid() { return node_; }
   u32 id();
   int use_count();
   void inc_use_count_unsafe();
 };
 
-const int kStackDepotMaxUseCount = 1U << 20;
+const int kStackDepotMaxUseCount = 1U << (SANITIZER_ANDROID ? 16 : 20);
 
 StackDepotStats *StackDepotGetStats();
 u32 StackDepotPut(StackTrace stack);
 StackDepotHandle StackDepotPut_WithHandle(StackTrace stack);
 // Retrieves a stored stack trace by the id.
 StackTrace StackDepotGet(u32 id);
 
 void StackDepotLockAll();

compiler-rt/trunk/lib/sanitizer_common/sanitizer_stackdepot.cc

 struct StackDepotNode {
   StackDepotNode *link;
   u32 id;
   atomic_uint32_t hash_and_use_count; // hash_bits : 12; use_count : 20;
   u32 size;
   u32 tag;
   uptr stack[1];  // [size]
 
-  static const u32 kTabSizeLog = 20;
+  static const u32 kTabSizeLog = SANITIZER_ANDROID ? 16 : 20;
   // Lower kTabSizeLog bits are equal for all items in one bucket.
   // We use these bits to store the per-stack use counter.
   static const u32 kUseCountBits = kTabSizeLog;
   static const u32 kMaxUseCount = 1 << kUseCountBits;
   static const u32 kUseCountMask = (1 << kUseCountBits) - 1;
   static const u32 kHashMask = ~kUseCountMask;
 
   typedef StackTrace args_type;