[fuchsia] Tune PrimaryGroupSizeLog to reduce fragmentation.

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.

Reviewed By: Chia-hungDuan

Differential Revision: https://reviews.llvm.org/D140311