fix(pubsub): fewer default threads for 32-bit builds

[coryan]

When the code is compiled for 32-bit platforms using 4 * thread-count background threads can quickly exhaust the address space. Reduce the default number of background threads to just 4.

Part of the work for #10775

---

This change is 

[codecov]

Codecov Report

Base: 93.65% // Head: 93.64% // Decreases project coverage by -0.01% ⚠️

Coverage data is based on head (67cde28) compared to base (f4fe373).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10793      +/-   ##
==========================================
- Coverage   93.65%   93.64%   -0.01%     
==========================================
  Files        1710     1710              
  Lines      153804   153804              
==========================================
- Hits       144038   144037       -1     
- Misses       9766     9767       +1     

Impacted Files	Coverage Δ
google/cloud/pubsub/internal/defaults.cc	100.00% <ø> (ø)
...te/integration_tests/publisher_integration_test.cc	92.10% <0.00%> (-6.58%)	⬇️
...le/cloud/internal/default_completion_queue_impl.cc	96.59% <0.00%> (-0.57%)	⬇️
...le/cloud/storage/internal/curl_download_request.cc	88.96% <0.00%> (ø)
google/cloud/completion_queue_test.cc	97.32% <0.00%> (+0.19%)	⬆️
...bigtable/examples/bigtable_hello_instance_admin.cc	83.00% <0.00%> (+2.00%)	⬆️
google/cloud/internal/async_connection_ready.cc	93.61% <0.00%> (+4.25%)	⬆️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.

[devbww]

s/thread-count/hardware-concurrency/ in the commit message.