Reduce TLAB size

Prevent pathological cases like threads fighting for TLABs and doing
back to back GC for alloc each allocation.

No performance change on angler, before:
EEAC: 1161, 1171, 1211
MemAllocTest: 1005, 971, 1148

After:
EEAC: 1173, 1175, 1163
MemAllocTest: 1003, 908, 803

There is also a mysterious speed up on volantis:
EEAC: 1563, 1553, 1543
MemAllocTest: 1474, 1486, 1485

After:
EEAC: 709, 714, 709
MemAllocTest: 664, 684, 655

Bug: 28976163

Change-Id: I564908badf395b6f948aa2dafcb66fe353fc1eba