cmd/6g: teach componentgen about string constants

[It's not clear that this is a significant net win. I'd expect the benchmarks to be better. Fixing issue 8525 first might help.]

Not ready for review or submission. Needs 8g, 5g, nacl. Those are blocked on the submission of CL 102890046.

Update issue 1914
Update issue 7384

This CL changes generated code for copying string literals from

LEAQ	go.string."hi"+0(SB),BX
MOVQ	(BX),DX
MOVQ	8(BX),CX

to

MOVQ	$go.string."hi"+16(SB),DX
MOVQ	$2,CX

This reduces the number of LEAQs in godoc from 45,229 to 30,227.

Change-Id: I12552a66041dffce5b5103f9b1c6b064ddd60fe0