Cleanup memset implementation. Similar to memcpy/memmove, use an offset and
bytes throughout. Simplify the complex calculations when optimizing for size
by using a fixed limit.
gcc/ChangeLog:
* config/aarch64/aarch64.cc (MAX_SET_SIZE): New define.
(aarch64_progress_pointer): Remove function.
(aarch64_set_one_block_and_progress_pointer): Simplify and clean up.
(aarch64_expand_setmem): Clean up implementation, use byte offsets,
simplify size calculation.