[dev.cc] liblink, cmd/internal/obj: fix printing of TYPE_REGREG and TYPE_REGREG2

[dev.cc] liblink, cmd/internal/obj: fix printing of TYPE_REGREG and TYPE_REGREG2

Now:

	0x0000 00000 (/tmp/x.s:2)	MULLU	R6,R3,(R7, R6)

The space is a little odd but I'd rather fix the usual printing to add spaces
than delete that one. But in a different CL, once C is gone.

Change-Id: I344e0b06eedaaf53cd79d370fa13c444a1e69c81
Reviewed-on: https://go-review.googlesource.com/4647
Reviewed-by: Rob Pike <r@golang.org>