Previously rb_ractor_interrupt_exec would use an intermediate function
to create a new thread with the actual target function, replacing the
data being passed in with a piece of malloc memory holding the "next"
function and the original data.
Because of this, passing rb_interrupt_exec_flag_value_data to
rb_ractor_interrupt_exec didn't have the intended effect of allowing
data to be passed in and marked.
This commit adds a rb_interrupt_exec_flag_new_thread flag, which
both simplifies the implementation and allows the original data to be
marked.
a34fcf401b