LD: Assert when CONFIG_PRIVILEGED_STACK_TEXT_AREA is too small
LD: Assert when CONFIG_PRIVILEGED_STACK_TEXT_AREA is too small

The error message is obscure when CONFIG_PRIVILEGED_STACK_TEXT_AREA is
configured incorrectly. It looks like this:

zephyr/linker.cmd:73 cannot move location counter backwards (from
0000000000069bfc to 0000000000069bd8)

This patch re-writes the linker script mechanism in a (believed and
tested) semantically equivalent way such that it is possible to use an
assertion.

The assertion's error message now looks like this:

Memory region         Used Size  Region Size  %age Used
           FLASH:      503012 B         1 MB     47.97%
            SRAM:       53760 B       256 KB     20.51%
        IDT_LIST:         120 B         2 KB      5.86
real-ld: The configuration system has incorrectly set
'CONFIG_PRIVILEGED_STACK_TEXT_AREA' to
128, which is not big enough. You must
through Kconfig either disable 'CONFIG_USERSPACE', or set
'CONFIG_PRIVILEGED_STACK_TEXT_AREA' to a value larger than
128 .
collect2: error: ld returned 1 exit status

Signed-off-by: Sebastian Bøe <edee888afd63e94916688495334700803eba20a0@nordicsemi.no>

explora26/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,nashif/zephyr,ldts/zephyr,nashif/zephyr,nashif/zephyr,Vudentz/zephyr,ldts/zephyr,explora26/zephyr,Vudentz/zephyr,finikorg/zephyr,ldts/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,explora26/zephyr,ldts/zephyr,galak/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,finikorg/zephyr,finikorg/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,galak/zephyr,Vudentz/zephyr,nashif/zephyr