for GCC4.6: fix a bunch of set-and-not-used errors.

src/cmd/6g/reg.c

src/cmd/6l/asm.c

src/cmd/6l/obj.c

src/cmd/gc/dcl.c

src/cmd/gc/reflect.c

src/cmd/gc/subr.c

src/cmd/gc/swt.c

src/cmd/gc/walk.c

src/cmd/ld/data.c

src/cmd/ld/ldelf.c

src/cmd/ld/lib.c

src/libmach/executable.c