Small improvements to run_spec_clang_asan.sh.

* update docs
* add some option verification
* add GCC support
* disable leak detection

Patch by Yuri Gribov.



git-svn-id: http://address-sanitizer.googlecode.com/svn/trunk@2208 3c3640eb-14a6-2bb8-0b94-e9e73afe33f5