6g: simplify trampoline by postponing load.
6g: simplify trampoline by postponing load.

	TEXT tramp
		MOVQ 8(SP), AX
		ADDQ $40, AX
		MOVQ AX, 8(SP)
		JMP oldfunc

	is now

	TEXT tramp
		ADDQ $40, 8(SP)
		JMP oldfunc

	and if s/40/0/, then it simplifies to

	TEXT tramp
		JMP oldfunc

	(the tramp is still needed to satisfy
	symbol references from other object files)

R=ken
OCL=28377
CL=28381

tav/go,tav/go,tav/go,tav/go,tav/go,tav/go,tav/go