cmd/internal/obj/riscv: rewrite branches
cmd/internal/obj/riscv: rewrite branches

As a first step in progedit, mark branching instructions as branches, as
other arches do.

Change-Id: Ia8e8dbb52ace860d3e461d76b5e7ef9ecbbdff6c

mgyenik/go,mgyenik/go,mgyenik/go,mgyenik/go,mgyenik/go,mgyenik/go,mgyenik/go,mgyenik/go