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LSym may be created concurrently. Reading/writing LSym.Pkg may
cause data race (see
https://build.golang.org/log/f0351c5cc7bf4c92e3aa5e78e294c2d009ebf118).
Fix this by setting LSym.Pkg only when holding the lock.

Change-Id: Ib3160ecf47c4ca530b09369e0e8284db6597cfd0
Reviewed-on: https://go-review.googlesource.com/c/go/+/198492

Reviewed-by: Than McIntosh <thanm@google.com>