[compiler-rt] suppress LNK4221 warning when creating static libraries

Summary:
We put empty object files in archives, which causes MSVC's linker to
complain about these objects not defining any previously undefined
symbols. Since we do it on purpose, this only creates noise during
the build process. This patch causes us to suppress the warnings.

Reviewers: rnk, samsonov

Subscribers: dberris, mgorny

Differential Revision: https://reviews.llvm.org/D28262

llvm-svn: 291011