aarch64: correct CFI in rawmemchr (bug 31113)

The .cfi_return_column directive changes the return column for the whole
FDE range.  But the actual intent is to tell the unwinder that the value
in x30 (lr) now resides in x15 after the move, and that is expressed by
the .cfi_register directive.

(cherry picked from commit 3f798427)