Use CMake's GNUInstallDirs

Description

This addresses https://gitlab.arm.com/networking/ral/-/issues/1 and changes our CMake to use a standard CMake module to set installation locations.

Checklist

• Contribution meets RAL's licence terms
• [] Documentation updated
• [] "Unreleased" section of the Changelog updated
• [] clang-format and clang-tidy run and changes included (C/C++ code)
• [] flake8 run and changes included (Python code)
• cmake-format run and changes included (CMake code)
• [] Commit message includes information on how to reproduce the issue(s)
• [] Tests added or updated
• [] Tests pass when run with AddressSanitizer

This change only affects the CMake files, so there is no need to update tests or documentation.