Do not require libatomic on FreeBSD

f3e09f36 hooked a dependency on libatomic on 32 bit machines if Clang is
used because compilation failed with:

`std::__atomic_base<unsigned long long>::load(std::memory_order) const':
/usr/bin/../lib/gcc/i686-pc-linux-gnu/6.1.1/../../../../include/c++/6.1.1/bits/atomic_base.h:396:
undefined reference to `__atomic_load_8'

But that does not happen on FreeBSD. The problem is likely that on Linux
Clang tries to use GCC-provided C++11 library. Further,
__atomic_load_8() (for 8-byte integers) is not readily available on 32
bit machines. From https://gcc.gnu.org/wiki/Atomic/GCCMM: "When lock
free instructions are not available (either through hardware or OS
support) atomic operations are left as function calls to be resolved by
a library."

CMakeLists.txt

@@ -698,7 +698,7 @@ endif()
 if(ANDROID)
   set(ATOMIC libatomic.a)
 endif()
-if(CMAKE_C_COMPILER_ID STREQUAL "Clang" AND ARCH_WIDTH EQUAL "32" AND NOT IOS)
+if(CMAKE_C_COMPILER_ID STREQUAL "Clang" AND ARCH_WIDTH EQUAL "32" AND NOT IOS AND NOT FREEBSD)
   find_library(ATOMIC atomic)
   list(APPEND EXTRA_LIBRARIES ${ATOMIC})
 endif()