cmake: Normalize source paths via -ffile-prefix-map for consistent builds

Applied -ffile-prefix-map to all bare-metal toolchain files (GNU, Clang,
ARMClang) to normalize source and build paths at compile time.

This change ensures:
- Consistent and relative paths in debug info and __FILE__ macros
- Cleaner objdump output and improved compatibility with coverage tools

Signed-off-by: Wei-Chia Su <Wei-Chia.Su@arm.com>