cmd/6l: accept NOP of $x+10(SP) and of X0

Needed to link code compiled with 6c -N.

R=ken2
CC=golang-dev
https://codereview.appspot.com/10043044
cmd/6c: use full 64-bit address in block copy

Already fixed independently in Plan 9.

R=ken2
CC=golang-dev
https://codereview.appspot.com/10041044