Simplify urandom and move iOS source into its own file

[torben-hansen]

Description of changes:

Refactor urandom.c to be more readable:

• Introduces explicit states
• Factor out code into self-contained functions
• Removes if/defs, in particular, drops any FIPS macro's
• Factors out iOS source CCRandomGenerateBytes() into its own file.

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

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 35.86957% with 59 lines in your changes missing coverage. Please review.

Project coverage is 78.70%. Comparing base (51233d4) to head (c1900ff).

Files with missing lines	Patch %	Lines
crypto/rand_extra/urandom.c	37.07%	56 Missing ⚠️
crypto/fipsmodule/rand/rand_isolated_test.cc	0.00%	3 Missing ⚠️

Additional details and impacted files

@@                    Coverage Diff                    @@
##           randomness_generation    #2379      +/-   ##
=========================================================
- Coverage                  78.74%   78.70%   -0.05%     
=========================================================
  Files                        637      637              
  Lines                     109341   109382      +41     
  Branches                   15494    15507      +13     
=========================================================
- Hits                       86099    86087      -12     
- Misses                     22519    22568      +49     
- Partials                     723      727       +4     

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[torben-hansen]

All these failures are known errors and some because fixes does not exist on target branch yet.