Remove darwin_fat.mk.

Summary:
Its seems to be replaced by clang_darwin.mk in the Makefile-based
build, and is only referenced in unittest scripts, which are
broken for a long time now.

Test Plan: n/a

Reviewers: bob.wilson

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6574

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@224946 91177308-0d34-0410-b5e6-96231b3b80d8