Don't call computeHostNumPhysicalCores when LLVM_ENABLE_THREADS is off

Summary:
Fix change from 8404aeb5 to avoid calling
computeHostNumPhysicalCores if LLVM_ENABLE_THREADS is off.

Reviewers: rnk, aganea

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D74654