[libc] Refactor sqrt implementations and add tests for generic sqrt implementations.

Refactor sqrt implementations:
- Move architecture specific instructions from `src/math/<arch>` to `src/__support/FPUtil/<arch>` folder.
- Move generic implementation of `sqrt` to `src/__support/FPUtil/generic` folder and add it as a header library.
- Use `src/__support/FPUtil/sqrt.h` for architecture/generic selections.
- Add unit tests for generic implementation of `sqrt`.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D118173