tsan: use smaller trace parts for Go

tsan: use smaller trace parts for Go

In the old runtime we used to use different number of trace parts
for C++ and Go to reduce trace memory consumption for Go.
But now it's easier and better to use smaller parts because
we already use minimal possible number of parts for C++ (3).

Reviewed By: melver

Differential Revision: https://reviews.llvm.org/D113978