flash/nor/stm32xx: fix segfault accessing Cortex-M part number

flash/nor/stm32xx: fix segfault accessing Cortex-M part number

Some of STM32 flash drivers read Cortex-M part number from
cortex_m->core_info.
In corner cases the core_info pointer was observed uninitialised
even if target_was_examined() returned true. See also [1]

Use the new and safe helper to get Cortex-M part number.

While on it switch also target_to_cm()/target_to_armv7m() to the safe
versions. This prevents a crash when the flash bank is misconfigured
with non-Cortex-M target.

Add missing checks for target_was_examined() to flash probes.

[1] 6545: fix crash in case cortex_m->core_info is not set
    https://review.openocd.org/c/openocd/+/6545

Change-Id: If2471af74ebfe22f14442f48ae109b2e1bb5fa3b
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Fixes: f5898bd93ff8 (flash/stm32fxx.c: do not read CPUID as this info is stored in cortex_m_common)
Reviewed-on: https://review.openocd.org/c/openocd/+/6752
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>