Adds bootstrap rules to support installing rust-demangler.

When compiling with `-Z instrument-coverage`, the coverage reports are
generated by `llvm-cov`. `llvm-cov` includes a built-in demangler for
C++, and an option to supply an alternate demangler. For Rust, we have
`rust-demangler`, currently used in `rustc` coverage tests.

Fuchsia's toolchain for Rust is built via `./x.py install`. Fuchsia is
adding support for Rust coverage, and we need to include the
`rust-demangler` in the installed `bin` directory.

Configured rust-demangler as an in-tree extended tool.

Added tests to support `./x.py test rust-demangler`.

Install with extended tools by default only if `profiler = true`.