strcmp: core: use movsx instead dirty code

src/ft_strcmp.asm

@@ -17,9 +17,6 @@ loop:
         inc rcx
         jmp loop
 out:
-        mov rax, 0
+        sub bl, dl
-        mov rbx, 0
+        movsx rax, bl
-        mov bl, BYTE [rsi + rcx]
-        mov al, BYTE [rdi + rcx]
-        sub rax, rbx
         ret