Impose a minimum tcache count for small size classes.

Now that small allocation runs have fewer regions due to run metadata
residing in chunk headers, an explicit minimum tcache count is needed to
make sure that tcache adequately amortizes synchronization overhead.

Bug: 21326736
(cherry picked from commit 83d543f8689bc7c6142179a5491bdf2a31b5cfc7)

Change-Id: I4178902b63ed310100019fee0805a11839de740f
2 files changed