arch: riscv: added helper config to include isr wrapper

Added helper Kcoinfig option USE_ISR_WRAPPER which can be used to
include isr_wrapper even if GEN_SW_ISR_TABLE is not enabled. This
is needed to enable configurations where only IRQ vector table is
used with multithreading (only direct isr used). This change is
backward compatibible with previous config.

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