[libc][stdfix] Initial support for stdfix.h and fixed point arithmetic.

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This is just a draft to see that what we need to build and test functions using _Fract and _Accum types. Will split them into smaller patches to commit.

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Build and test with x86-64 clang (trunk) works with latest version.

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Spinoff 1: First patch to generate stdfix.h header and add compiler support detection #81255

Spinoff 2: Add is_fixed_point type trait #81263

Spinoff 3: Add FXRep helper class for fixed point types #81272

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Spinoff 4: Add support for fixed point types in the testing infrastructure. #81819
Spinoff 5: Add abs functions for signed fixed point types. #81823