doc: make sure we use gdb from SDK
doc: make sure we use gdb from SDK

installed GDB on host might not be compatible with the binary generated
by Zephyr. Mention that we need to use the gdb that corrosponds to the
toolchain being used.

Fixes #4312

Signed-off-by: Anas Nashif <0d9952ec84ac43c159f6b7e7ed99a9080c00dd6e@intel.com>

aceofall/zephyr-iotos,finikorg/zephyr,Vudentz/zephyr,nashif/zephyr,mbolivar/zephyr,explora26/zephyr,Vudentz/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,aceofall/zephyr-iotos,galak/zephyr,punitvara/zephyr,ldts/zephyr,ldts/zephyr,finikorg/zephyr,explora26/zephyr,punitvara/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,mbolivar/zephyr,nashif/zephyr,ldts/zephyr,zephyriot/zephyr,zephyriot/zephyr,kraj/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,explora26/zephyr,explora26/zephyr,finikorg/zephyr,punitvara/zephyr,finikorg/zephyr,nashif/zephyr,mbolivar/zephyr,finikorg/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,mbolivar/zephyr,explora26/zephyr,Vudentz/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,aceofall/zephyr-iotos,kraj/zephyr,galak/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,zephyriot/zephyr,kraj/zephyr,aceofall/zephyr-iotos,kraj/zephyr,mbolivar/zephyr,GiulianoFranchetto/zephyr,zephyriot/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,zephyriot/zephyr,aceofall/zephyr-iotos,Vudentz/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr