x86-64 machine code, 17 bytes
B0 41 AA 04 01 3C 4C 74 FA 3C 5A 76 F5 C6 07 00 C3 Following the standard calling convention for Unix-like systems (from the System V AMD64 ABI), this takes in RDI a memory address at which to place the result, as a null-terminated byte string.
In assembly:
f: mov al, 'A' # Set AL to the first letter, 'A'. r: stosb # Write AL to the output string, advancing the pointer. b: add al, 1 # Add 1 to AL to get the next letter. cmp al, 'L' # Compare it to 'L'... je b # and if it's equal, jump back to add 1 again. cmp al, 'Z' # Compare it to 'Z'... jbe r # and if it's less than or equal, repeat the loop. mov BYTE PTR [rdi], 0 # Add a null terminator to the string. ret # Return.