compute_num_cpus() purports to clamp to 16, but doesn't.
Our compute_num_cpus() code is not supposed to report more than MAX_DETECTABLE_CPUS, since if you are somehow running Tor on a 64-core system, you should have to ask Tor before it decides to use all 64.
But it doesn't: the code logs a notice, and then does nothing.
This causes the following test failure on systems with more than 16 cores:
util/num_cpus:
FAIL ../src/test/test_util.c:3827: assert(num OP_LE 16): 18 vs 16
029 rationale: this is apparently making our tests fail in the debian reproducible build QA system.