builtins: add missing file

Add (missing) definition of COMPILER_RT_EXPORT which is meant to be used for
decorating functions that are meant to be exported.  This is useful for
platforms where exports and imports must be decorated explicitly (i.e. Windows).

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@208593 91177308-0d34-0410-b5e6-96231b3b80d8