Zstdmt count wrong number of cores

On my i7-6850K zstdmt claims there is only 6 cores available. But cpuinfo shows 12 cores, and `siblings`, command `grep -c processor /proc/cpuinfo` also shows 12.

**To Reproduce**
Steps to reproduce the behavior:
1. Install zstd v1.5.7; `apt install zstd
2. Run `zsrdmt with flags -14 -v
3. Scroll up on the log to 'Note: 6 physical core(s) detected'
4. See error

**Expected behavior**
Number must be equal to number of siblings or logical coree.

**Screenshots and charts**
If applicable, add screenshots and charts to help explain your problem.

**Desktop (please complete the following information):**
- OS: Debian 7+
- Version:1.5.7
- Compiler: gcc 4.7.2
- Flags: -O2
- Other relevant hardware specs: 6-core
- Build system: Debian default