selftests/ftrace: Do not use arch dependent do_IRQ as a target function
Instead using arch-dependent do_IRQ, use do_softirq as a
target function.
Applying do_IRQ to set_ftrace_filter always fail on arm/arm64 and any
other architectures which don't define do_IRQ. So, instead of using
that, use do_softirq which is defined in kernel/softirq.c.