Apply Hardened compiler flags to CMake

- Introduced `CXX_FLAGS` and `LINKER_FLAGS` for Hardened compiler and linker options.
- Added `apply_flags()` function to enforce compiler and linker flags per target.
- Marked third-party includes (`half/include`) as `SYSTEM PUBLIC` to suppress warnings.

Signed-off-by: Yuvaraj Venkatesh <yuvaraj.venkatesh@arm.com>
Change-Id: I1b1e06f9bdd7c10d87fba31d6f25d9fa13f072be