Why does the header file sys/syscall.h not include asm-x86_64/unistd.h in RHEL 6.4?

Issue

• In RHEl 6.4, the sys/syscall.h file has the format:

# ifdef __i386__
#  include "unistd_32.h"
# else
#  include "unistd_64.h"
# endif

• While other linux versions use:

#ifndef __ASM_STUB_UNISTD_H
#define __ASM_STUB_UNISTD_H
# if defined __x86_64__
#  include <asm-x86_64/unistd.h>
# elif defined __i386__
#  include <asm-i386/unistd.h>
# else
#  warning This machine appears to be neither x86_64 nor i386.
# endif
#endif /* __ASM_STUB_UNISTD_H */