cmd/5l: support -Z (zero stack frame at function entry)
also added appropriate docs to cmd/ld/doc.go
(largely copied from Russ's CL 6938073).

R=rsc
CC=golang-dev
https://codereview.appspot.com/7004049
cmd/5g: allow optimization of return registers.
Modeled after CL 7030046 by daniel.morsing.

example program:
func f(x int) int { x -= 10; return x }

5g -S difference:
 --- prog list "f" ---
 0011 (x.go:7) TEXT   	add+0(SB),$0-8
 0012 (x.go:7) MOVW   	x+0(FP),R0
-0013 (x.go:7) SUB    	$10,R0,R2
-0014 (x.go:7) MOVW   	R2,R0
-0015 (x.go:7) MOVW   	R2,.noname+4(FP)
-0016 (x.go:7) RET    	,
+0013 (x.go:7) SUB    	$10,R0
+0014 (x.go:7) MOVW   	R0,.noname+4(FP)
+0015 (x.go:7) RET    	,

R=dave, rsc
CC=golang-dev
https://codereview.appspot.com/7030047