Ensure that only one compiler-rt component is created for lsan

Summary:
The lsan cmake configuration failed when targeting more
than one architecture, because it would attempt to create multiple
components with the same name. Ensure that only one lsan component
is ever created.

Reviewers: beanz, bogner

Subscribers: dberris, llvm-commits, mgorny

Differential Revision: https://reviews.llvm.org/D28151

llvm-svn: 291294