[fuchsia] Tune PrimaryGroupSizeLog to reduce fragmentation.

[supersymetrie]

Summary

21U is the default group size, which demonstrates ~15mb reduction
in heap size for some highly fragmented heaps on Fuchsia, and
a general 5mb savings when devices are under no load.

Microbenchmarks show no performance regressions, but most of our
benchmarks perform no significant mallocs. So we are choosing the
default setting, and monitoring for potential performance
issues.

[supersymetrie]

After speaking with Android scudo maintainters, they feel
that 128mb primary group size is more conservative than need be,
and the default size should be sufficient.

compiler-rt/lib/scudo/standalone/allocator_config.h

 
 #if SCUDO_CAN_USE_PRIMARY64
 struct FuchsiaConfig {
   using SizeClassMap = FuchsiaSizeClassMap;
   static const bool MaySupportMemoryTagging = false;
 
   typedef SizeClassAllocator64<FuchsiaConfig> Primary;
   static const uptr PrimaryRegionSizeLog = 30U;
-  static const uptr PrimaryGroupSizeLog = 30U;
+  static const uptr PrimaryGroupSizeLog = 27U;
   typedef u32 PrimaryCompactPtrT;
   static const bool PrimaryEnableRandomOffset = true;
   static const uptr PrimaryMapSizeIncrement = 1UL << 18;
   static const uptr PrimaryCompactPtrScale = SCUDO_MIN_ALIGNMENT_LOG;
   static const s32 PrimaryMinReleaseToOsIntervalMs = INT32_MIN;
   static const s32 PrimaryMaxReleaseToOsIntervalMs = INT32_MAX;
 
   typedef MapAllocatorNoCache SecondaryCache;