Update CMake to 3.9

[waahm7]

Issue #, if available:

• https://github.com/awslabs/aws-c-common/issues/1142

Description of changes:

• set visibility always to hidden
• Update CMake to 3.9

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 83.62%. Comparing base (faa6c0f) to head (21880c4). :warning: Report is 44 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1159      +/-   ##
==========================================
- Coverage   83.84%   83.62%   -0.23%     
==========================================
  Files          57       57              
  Lines        5992     5947      -45     
==========================================
- Hits         5024     4973      -51     
- Misses        968      974       +6     

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