Added launching scripts.

* template/x86_64-unknown-linux-gnu/site-config.jam:
  * Added support of launching scripts.
  * Removed fine-grained library configuration based on `LD_PRELOAD`.
* template/x86_64-unknown-linux-gnu/intro-launcher: Newly added.
* template/x86_64-unknown-linux-gnu/catchsegvfilt: Newly added.
* template/x86_64-unknown-linux-gnu/asanlogfilt: Newly added.
* gcc/jamfile: Added installation of launching scripts.