cgen,cheaders: move _vinit_caller()/_vcleanup_caller() fn declare forword

[kbkpbot]

Fix issue #23496

This PR will make V's dll can be called by other program.
But the v function should call _vinit_caller()/_vcleanup_caller() directly....

example:

fn C._vinit_caller()
fn C._vcleanup_caller()

@[export: 'func']
fn func() {
	C._vinit_caller()          // must call first
	println('Begin DLL')
	for i in 0 .. 10 {
		println('${i}')
	}
	println('End DLL')
	C._vcleanup_caller()    // must call last,  note, currently , it is an empty function....
}

[huly-for-github]

Connected to Huly®: V_0.6-21936

[spytheman]

(the CI failure is unrelated and already fixed on master)

[spytheman]

I fail to see, what the benefit of this is, compared to the previous approach, of only generating those lines conditionally.