arch: arm: cortex-m: fix MPU workaround

The workaround for ARMv7-M architecture (which proactively
decreases the available thread stack by the size of the MPU
guard) needs to be placed before we calculate the pointer of
the user-space local thread data, otherwise this pointer will
fall beyond the boundary of the thread stack area.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>