[CMake][libcxx] Link libc when checking for atomics support

[phosek]

Summary

The C library is needed when linking the atomics test executable.

[phosek]

This doesn't seem to be needed anymore with D31617.

cmake/Modules/CheckLibcxxAtomic.cmake

 INCLUDE(CheckCXXSourceCompiles)
 
 # Sometimes linking against libatomic is required for atomic ops, if
 # the platform doesn't support lock-free atomics.
 #
 # We could modify LLVM's CheckAtomic module and have it check for 64-bit
 # atomics instead. However, we would like to avoid careless uses of 64-bit
 # atomics inside LLVM over time on 32-bit platforms.
 
 function(check_cxx_atomics varname)
   set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
-  set(CMAKE_REQUIRED_FLAGS "-nodefaultlibs -std=c++11 -nostdinc++ -isystem ${LIBCXX_SOURCE_DIR}/include")
+  set(CMAKE_REQUIRED_FLAGS "-nodefaultlibs -std=c++11 -nostdinc++ -isystem ${LIBCXX_SOURCE_DIR}/include -lc")
   if (${LIBCXX_GCC_TOOLCHAIN})
     set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} --gcc-toolchain=${LIBCXX_GCC_TOOLCHAIN}")
   endif()
   if (CMAKE_C_FLAGS MATCHES -fsanitize OR CMAKE_CXX_FLAGS MATCHES -fsanitize)
     set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -fno-sanitize=all")
   endif()
   if (CMAKE_C_FLAGS MATCHES -fsanitize-coverage OR CMAKE_CXX_FLAGS MATCHES -fsanitize-coverage)
     set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -fno-sanitize-coverage=edge,trace-cmp,indirect-calls,8bit-counters")