This changes the bootstrap config `target.*.profiler` from a plain bool
to also allow a string, which will be used as a path to the pre-built
profiling runtime for that target. Then `profiler_builtins/build.rs`
reads that in a `LLVM_PROFILER_RT_LIB` environment variable.
All other Platform files included in `llvm-project/compiler-rt` were
present, except Fuchsia.
Now that there is a functional end-to-end version of
`-Zinstrument-coverage`, I need to start building and testing
coverage-enabled Rust programs on Fuchsia, and this file is required.
