cmd/internal/obj/riscv: simplify machine code output

Use cursym.WriteInt rather than building up a slice of bytes and then writing them
out via PutUint32. This also allows for variable instruction sizes, which will be
needed when support for compressed (2 byte length) instructions is added.

Change-Id: I17c9ffa52d27c91a24e161317e3db28e224804ae
Reviewed-on: https://go-review.googlesource.com/c/go/+/344460


Trust: Joel Sing <joel@sing.id.au>
Reviewed-by: Cherry Mui <cherryyz@google.com>