Export compile commands (via ninja -t compdb) when building the runtime

[rdw-software]

Goals:

• The build system generates clang-compatible JSON files that can be used to fix annoying squiggles in vscode

compdb
Given a list of rules, each of which is expected to be a C family language compiler rule whose first input is the name of the source file, prints on standard output a compilation database in the JSON format expected by the Clang tooling interface. Available since Ninja 1.2.