Use std::endl to flush Stan prints

[WardBrian]

Submission Checklist

• [x] Run unit tests
• Documentation
  • [ ] If a user-facing facing change was made, the documentation PR is here:
  • [x] OR, no user-facing changes were made

Release notes

When printing we currently just append a \n character, but this will not always cause the print to flush.

Copyright and Licensing

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the BSD 3-clause license (https://opensource.org/licenses/BSD-3-Clause)

[codecov[bot]]

Codecov Report

Merging #1301 (ef251a1) into master (a490a09) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1301   +/-   ##
=======================================
  Coverage   89.02%   89.03%           
=======================================
  Files          64       64           
  Lines        9752     9754    +2     
=======================================
+ Hits         8682     8684    +2     
  Misses       1070     1070           

Impacted Files	Coverage Δ
src/stan_math_backend/Cpp.ml	86.70% <100.00%> (+0.07%)	:arrow_up:
src/stan_math_backend/Lower_stmt.ml	96.84% <100.00%> (ø)