atomic: undef macros that are included from atomic_decl.inc

Signed-off-by: Aaron Watry <awatry@gmail.com>
Reviewed-by: Jeroen Ketema <j.ketema@imperial.ac.uk>

git-svn-id: https://llvm.org/svn/llvm-project/libclc/trunk@217958 91177308-0d34-0410-b5e6-96231b3b80d8