arch: riscv: option to init custom hw stacked esf members.

When RISCV_SOC_HAS_ISR_STACKING is used, it may
be needed to initialize custom hw stacked esf members.
Some initial values may need to be aligned with
hw stacking mechanism to avoid any side effects.

Signed-off-by: Łukasz Stępnicki <lukasz.stepnicki@nordicsemi.no>