* Clang: Use --version instead of -dumpversion

Fixes issue when Clang 8 and below determined as 4.2.1 version.

* Bootstrap and test with versioned toolset

Eliminates need to manual configure toolsets via `user-config.jam` for testing, and allows to pass to `bootstrap.sh` the same toolset string as you would to B2 invocation what eliminates need to specify a custom exec via `--cxx=`.