It's because you're using GCC 4.2 on your Mac.  It's not in GCC till 4.3.  

But should be cppd so that it's not used on x86 architectures so that this use doesn't happen 

On Monday, November 25, 2013, Amos Robinson wrote:
Sorry for the noise, it turns out there's already a trac ticket:
https://ghc.haskell.org/trac/ghc/ticket/8561
(I didn't find it because I was searching for ___builtin… instead of __builtin!)

On Tue, Nov 26, 2013 at 3:39 PM, Amos Robinson <amos.robinson@gmail.com> wrote:
> Hi,
> I'm having trouble linking with head:
>
>> [1 of 1] Compiling Main             ( bindisttest/HelloWorld.lhs, bindisttest/HelloWorld.o )
>> Linking bindisttest/HelloWorld ...
>> Undefined symbols for architecture x86_64:
>>   "___builtin___clear_cache", referenced from:
>>      _flushExec in libHSrts.a(Storage.o)
>> ld: symbol(s) not found for architecture x86_64
>> collect2: ld returned 1 exit status
>
> This is on OSX 10.8.5, where gcc --version gives me
>> i686-apple-darwin11-llvm-gcc-4.2 (GCC) 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2336.11.00)
>
> I get the impression that this commit is related. Is it the case that
> Apple switched from gcc to clang recently? Could that be why
> ___builtin__clear_cache is missing?
>
> I'm sorry, I don't really know anything about the runtime system, so
> am kind of stuck.
>
>
--SNIP--
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