cmd/link: start file-local symbols at version 10

We're going to use the linker's symbol versions to track ABIs.
Currently, version 0 is used for global symbols and version > 0 is
used for file-local symbols. This CL reserves versions 0 to 9 for
global symbols with ABIs and uses version 10 and up for file-local
symbols. To make this clean, it also introduces a method on Symbol for
querying whether it's file-local.

For #27539.

Change-Id: Id3bc7369268f35128b14318a62e86335181a80e5
Reviewed-on: https://go-review.googlesource.com/c/146859


Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>