sim/aarch64: Fix register ordering bug in blr (PR sim/25318)

sim/aarch64: Fix register ordering bug in blr (PR sim/25318)

A comment in the implementation of blr says:

  /* The pseudo code in the spec says we update LR before fetching.
     the value from the rn.  */

With 'rn' being the register holding the destination address.

This may have been true at one point, but the ISA manual now clearly
shows the destination register being read before the link register is
written.

This commit updates the implementation of blr to match.

sim/aarch64/ChangeLog:

	PR sim/25318
	* simulator.c (blr): Read destination register before calling
	aarch64_save_LR.

Change-Id: Icb1c556064e3d9c807ac28440475caa205ab1064