target: cortex_a: fix regs invalidation when -defer-examine

target: cortex_a: fix regs invalidation when -defer-examine

The code for cortex_a allocates the register cache during the very
first examine of the target.
To prevent a segmentation fault in assert_reset(), the call to
register_cache_invalidate() is guarded by target_was_examined().

But for targets with -defer-examine, the target is set as not
examined in handle_target_reset() just before entering in
assert_reset().

This causes registers to not be invalidated while reset a target
examined but with -defer-examine.

Change the condition and invalidate the register cache if it has
been already allocated.

Change-Id: I81ae782ddce07431d5f2c1bea3e2f19dfcd6d1ce
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8215
Tested-by: jenkins