Remove -latomic if unnecessary

[Matt Glazar]

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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