check if 64 or 32 bits using CS segment (64=33, 32=23)

64_or_32.S

// To try it in i386:
// as --32 -o cs.o cs.S && ld -melf_i386 -o cs cs.o
.section .text
.global _start
_start:
    movw %cs, %cx
    shr $4, %cx

    cmpb $3, %cl
    je x86_64
    jmp i386

x86_64:
    movb $60, %al
    movw $64, %di
    syscall

i386:
    movb $1, %al
    movw $32, %bx
    int $0x80