CMake changes: Long double/double size detection, Android x86 and armv7 support, install pkg-config pc file

[ViliusSutkus89]

Hello,

This PR has the following changes:

• Generate pkg-config openlibm.pc file (issue CMake parity with Makefile #301)
• Support for Android armv7-a and Android x86
• Reworked long double size detection. Previously it was hardcoded, I've added a test which compares LDBL_MANT_DIG and DBL_MANT_DIG from float.h. Previously hardcoded cases did not cover Android x86 correctly. A simple compiler check should produce a reliable result.

Also, when we release, we need to update version number at the top of CMakeLists.txt file, otherwise the generated pkg-config openlibm.pc file will report the wrong version. I've set the version to 0.8.4 in advance. I don't really care about it, because I don't use pkg-config.pc file, I'm just saying what would be the consequences of not incrementing the version number.
Where do we set the version number for pkg-config.pc file generated by autotools?

Once again, #299 will require a merge commit, because it's the same file we are editing, but we touched different lines, so we should be good.

Regards

[ViralBShah]

@ViliusSutkus89 Happy to give you commit access here. Please let me know if ok.

[ViliusSutkus89]

Sure. I can't be a full time maintainer, but I'll try to help whenever I can

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 39.02%. Comparing base (4b83beb) to head (c1e8462).
Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #304      +/-   ##
==========================================
+ Coverage   38.24%   39.02%   +0.78%     
==========================================
  Files         233      233              
  Lines        6139     6144       +5     
  Branches     1607     1608       +1     
==========================================
+ Hits         2348     2398      +50     
+ Misses       3394     3393       -1     
+ Partials      397      353      -44     

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[ViliusSutkus89]

Updated PR with a patch to match version in CMake with the version in Autotools