[Darwin] [Builtins] Remove multi3 from the exclude list.
[Darwin] [Builtins] Remove multi3 from the exclude list.

This addresses PR27077. For some historical reason Darwin wasn't shipping multi3 in the compiler builtin library or in the OS builtin library. This caused building ffmpeg to fail because Polly was generating calls to multi3. It is easy enough to just add the builtin.

git-svn-id: c199f293c43da69278bea8e88f92242bf3aa95f7@264750 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt