compile_commands.json omits -flto and rewrites the compiler as clang++ --target=avr #1467

While auditing cold-build perf, .fbuild/build/uno/release/compile_commands.json listed clang++ --target=avr with no -flto, although the real avr-g++ invocations include -flto. That misleads anyone comparing flags or replaying commands; I briefly thought fbuild didn't use LTO. Either emit the real command, or add the real one next to the clangd-oriented one (for example, a compile_commands.raw.json).