Before this, stack protection would be effective only after switching to
the first thread.
Even before the first thread is created, the kernel init code uses the
IRQ stack to set things up. Let's make sure this is safeguarded as well.
This also fixes the incompatibility between CONFIG_RISCV_PMP and
CONFIG_RISCV_ALWAYS_SWITCH_THROUGH_ECALL, the later needing an exception
call to switch to the first thread and exception code assuming stack
guard is already set up in the PMP.
Signed-off-by: Nicolas Pitre <npitre@baylibre.com>
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