cmd/internal/obj: delete all Pconv, replace with Prog.String

cmd/internal/obj: delete all Pconv, replace with Prog.String

Remove the per-achitecture formatter for Prog and replace it with
a global String method. Clean up and regularize the output. Update
tests affected by the format; some tests are made correct now when
they were broken before (and known to be).

Also, related: Change the encoding of the (R1+R2) syntax on ppc64
to be equivalent to (R1)(R2*1), which means it needs no special
handling.

Delete the now unused STRINGSZ constant.

Change-Id: I7f6654d11f80065f3914a3f19353f2f12edfe310
Reviewed-on: https://go-review.googlesource.com/6931
Reviewed-by: Russ Cox <rsc@golang.org>