OpenBLAS

cpp_thread_test/dgemv: cap concurrency to number of hw threads on small systems

... instead of (number of hw threads - 4) to avoid invalid numbers on
smaller systems. Currently, systems with 4 or fewer CPUs (e.g., small CI
VMs) would fail the test. Fixes one of the issues discussed in #2668

Signed-off-by: Marius Hillenbrand <mhillen@linux.ibm.com>