[NFC sanitizer_symbolizer] Move Fuchsia specific code.

Moves sanitizer symbolizer code that is specific for
fuchsia into its own _fuchsia.cpp file.
This is preparation to enable symbolizer markup in
linux.

Reviewers: PiJoules, petrhosek, vitalybuka

Reviewed By: PiJoules, vitalybuka

Pull Request: https://github.com/llvm/llvm-project/pull/73192