cmd/asm: stop using "" as the local package prefix

cmd/asm: stop using "" as the local package prefix

Now that cmd/asm always knows its package path, we can create symbols
with the appropriate package prefix instead of "".

Change-Id: I56864089e8f1b38ff4197b3158131f976b329572
Reviewed-on: https://go-review.googlesource.com/c/go/+/523336
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>