Simplify and cleanup build constants

[JimBobSquarePants]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

Description

As part of the move to externalize ImageSharp.Drawing and import Core I thought it wise to simplify and normalize our use of #ifdefs throughout the library.

Once we have a CI build environment that works on .NET Core 3.1 we can easily add additional target frameworks.

Touch #1002

[codecov]

Codecov Report

Merging #1075 into master will increase coverage by <.01%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master    #1075      +/-   ##
==========================================
+ Coverage   89.95%   89.95%   +<.01%     
==========================================
  Files        1200     1200              
  Lines       53013    53010       -3     
  Branches     3847     3847              
==========================================
- Hits        47689    47687       -2     
+ Misses       4556     4555       -1     
  Partials      768      768

Impacted Files	Coverage Δ
src/ImageSharp/Formats/Bmp/BmpDecoderCore.cs	94.38% <ø> (ø)	⬆️
src/ImageSharp/Formats/Bmp/BmpEncoderCore.cs	95.33% <ø> (ø)	⬆️
src/ImageSharp/Formats/Tga/TgaDecoderCore.cs	88.1% <ø> (ø)	⬆️
src/ImageSharp/Formats/Gif/LzwDecoder.cs	92.94% <ø> (ø)	⬆️
src/ImageSharp/Formats/Tga/TgaEncoderCore.cs	98.51% <ø> (ø)	⬆️
...c/ImageSharp/Common/Extensions/StreamExtensions.cs	93.33% <75%> (+4.44%)	⬆️

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[JimBobSquarePants]

Imma gonna go ahead with this now I've consolidated the build constants. I want to move on to #1076

[antonfirsov]

lgtm after a quick review on mobile