First steps with amd64 assembly
I'm not sure whether it's correct:


If show_hash uses cdecl calling conventions you should preserve registers like %bl, and read the input parameter values from the stack into esi and edi.
(see section "3.2.3 Parameter Passing" of the Application Binary Interface: parameters are conveniently passed in the rdi and rsi registers)
I don't understand the syntax of mov 1b(%rax),%al: is it reading from the lookup array defined at label 1:? On re-reading, I think it is; however that will only work if the most-significant bytes of rax and rdx are all zero; perhaps you should initialize them as you did using xor %eax,%eax
(apparently xor %eax,%eax will clear the whole of %rax)
You're using stosw which writes two bytes (two ASCII characters) a time; but %ah contains  a value from %bl, and %bl wasn't previously initialized? I think that statement should have been mov %dl,%ah not mov %bl,%ah; or perhaps you could have done mov 1b(%rdx),%ah directly.
The function ends with mov %cl,(%rdi) to null-terminate the string; that is clever but could use a comment (it took me a bit to figure out). Most of the assembly questions either have good comments, or answer which say that they should have good comments.