gc, 8g, 8l: fix a handful of warnings

gc, 8g, 8l: fix a handful of warnings

8g/cgen.c
        print format type mismatch

8l/asm.c
        resoff set and not used

gc/pgen.c
        misleading comparison INT > 0x80000000

gc/reflect.c
        dalgsym must be static to match forward declaration

gc/subr.c
        assumed_equal set and not used
        hashmem's second argument is not used

gc/walk.c
        duplicated (unreachable) code

R=rsc
CC=golang-dev
https://golang.org/cl/5651079