hwasan: Update register-dump-read.c test to reserve x23 instead of x20.

hwasan: Update register-dump-read.c test to reserve x23 instead of x20.

D90422 changed this test to write a fixed value into register x23
instead of x20, but it did not update the list of reserved registers.
This meant that x23 may have been live across the register write,
although this happens to not be the case with the current compiler.
Fix the problem by updating the reserved register list.