Kernel: Add 8-byte atomics for i686 GCC

Unlike Clang, GCC does not support 8-byte atomics on i686 with the
-mno-80387 flag set, so until that is fixed, implement a minimal set of
atomics that are currently required.

Kernel/CMakeLists.txt

@@ -362,6 +362,13 @@ if ("${SERENITY_ARCH}" STREQUAL "i686" OR "${SERENITY_ARCH}" STREQUAL "x86_64")
             ${CMAKE_CURRENT_SOURCE_DIR}/Arch/x86/${KERNEL_ARCH}/SyscallEntry.cpp
         )
     endif()
+
+    if ("${SERENITY_ARCH}" STREQUAL "i686")
+        set(KERNEL_SOURCES
+            ${KERNEL_SOURCES}
+            ${CMAKE_CURRENT_SOURCE_DIR}/Arch/x86/${KERNEL_ARCH}/Atomics.cpp
+        )
+    endif()
 endif()
 
 set(AK_SOURCES