[ASan] Optional support for dynamic ASan runtime on Linux.

Based on http://llvm-reviews.chandlerc.com/D3042 by Yuri Gribov!


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@205308 91177308-0d34-0410-b5e6-96231b3b80d8