target: aarch64: fix out-of-bound access to array

target: aarch64: fix out-of-bound access to array

The command 'arm core_state' uses the enum in 'arm->core_state' as
an index in the table of strings to print the core state.

With [1] the enum has been extended with the new state for AArch64
but not the corresponding table of strings.
This causes an access after the limit of arm_state_strings[].

Rewrite the table using c99 array designators to better show the
link between the enum list and the table.
Add the function arm_core_state_string() to check for out-of-bound
values allover the file.

Change-Id: I06473c2c8088b38ee07118bcc9e49bc8eafbc6e2
Fixes: [1] 9cbfc9feb35c ("arm_dpm: Add new state ARM_STATE_AARCH64")
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8594
Tested-by: jenkins