Fix OpenBSD build.

[devnexen]

Also in case fiber context stack creation via mmap is enabled, adding MAP_STACK flag for this platform.

[codecov]

Codecov Report

Merging #4108 (5813a97) into master (490ef86) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4108      +/-   ##
==========================================
+ Coverage   48.76%   48.77%   +0.01%     
==========================================
  Files          71       71              
  Lines       14269    14269              
==========================================
+ Hits         6958     6960       +2     
+ Misses       7311     7309       -2     

Impacted Files	Coverage Δ
src/coroutine/context.cc	92.59% <ø> (ø)
src/lock/mutex.cc	75.00% <ø> (ø)
src/os/async_thread.cc	68.85% <0.00%> (+0.87%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 490ef86...5813a97. Read the comment docs.