5g: fix build on Plan 9

5g: fix build on Plan 9

5g/cgen.c:
. USED(n4) as it is only mentioned in unreachable code later;
. dropped unused assignments;
. commented out unreachable code;
5g/cgen64.c:
5g/ggen.c:
. dropped unused assignments of function return value;
5g/gg.h:
. added varargck pragmas;
5g/peep.c:
. USED(p1) used only in unreacheable code;
. commented out unreachable code;
5g/reg.c:
. dropped unused assignment;

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/4953048