target/arm: make 'arm core_state' command compatible with Cortex-M

target/arm: make 'arm core_state' command compatible with Cortex-M

Tcl command 'arm core_state' was exposed even on Cortex-M devices.
However it returned message "Unsupported Command" without error status
on such device.

Set the only possible arm->core_state ARM_STATE_THUMB in armv7m init.
Block setting core_state to arm on Cortex-M.

Change-Id: I9525553ac8863a6cf77bbacbcd57e354b6cfe1ca
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: https://review.openocd.org/c/openocd/+/7100
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Tested-by: jenkins