Remove -latomic if unnecessary

[Chris Bieneman]

LGTM.

-Chris

> On Feb 23, 2015, at 1:48 PM, Matt Glazar <strager at fb.com> wrote:
> 
> cc Nakamura and Chandler, as suggested by abdulras at fb.com
> 
> -----Original Message-----
> From: Matt Glazer <strager at fb.com>
> Date: Tuesday, February 3, 2015 at 5:34 PM
> To: "llvm-commits at cs.uiuc.edu" <llvm-commits at cs.uiuc.edu>
> Subject: Remove -latomic if unnecessary
> 
>> My Linux toolchain supports __atomic_fetch_add_4 without -latomic.
>> Specifying -latomic fails because libatomic.{so,a} does not exist.
>> 
>> In CMake, first check if __atomic_fetch_add_4 can be used without
>> -latomic.  If it can't, try with -latomic.
>> 
>> See attached patch.
>> 
> 
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