Delete remaining compiler-rt makefiles

Summary:
Since we can now build the builtins without a full toolchain these files should no longer be needed.

This is the last vestige of autoconf!

Reviewers: compnerd, iains, jroelofs

Subscribers: dberris, llvm-commits

Differential Revision: https://reviews.llvm.org/D23777

llvm-svn: 279539