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.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>