Change windows macro for warnings into compiler flags

[peterfpeterson]

Reducing the contents of MantidKernel/System.h and the generated export headers should speed up mantid builds because the pre-compiler has less to parse. This migrates the 3 compiler waring suppressions to be compiler flags. It should have no effect on linux or osx which were already protected by the #ifdef macro.

Refs #38457

To test:

The builds passing is sufficient for this change to be considered correct. This is not user facing therefore does not require release notes.

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[jclarkeSTFC]

Builds fine on Windows, if anyone using Visual Studio is reading this PR, then the disabled warnings end up here:

[thomashampson]

I'm going to wait until the end of today to merge this because I know that there are a couple of Windows developers finishing things off and it would be inconvenient if they had to rebuild right now.