target/arm: do not expose 'arm reg', 'arm mcr/mrc' commands on Cortex-M

target/arm: do not expose 'arm reg', 'arm mcr/mrc' commands on Cortex-M

Tcl commands 'arm reg', 'arm mcr/mrc' do not work on M-profile based devices.
Isolate them from 'arm core_state' and 'arm disassemble' and do not chain
them from armv7m_command_handlers.

Change-Id: I2c6befdf82575e95cf05ed158ab5e6faa1a182c3
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: https://review.openocd.org/c/openocd/+/7101
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Tested-by: jenkins