[release-branch.go1.21] cmd/link: avoid deadcode of global map vars for programs using plugins
If a program imports the plugin package, the mechanisms in place for
detecting and deleting unused global map variables are no longer safe,
since it's possibly for a given global map var to be unreferenced in
the main program but referenced by a plugin. This patch changes the
linker to test for plugin use and to avoid removing any unused global
map variables if the main program could possibly load up a plugin.
Fixes #62505.
Updates #62430.
Change-Id: Ie00b18b681cb0d259e3c859ac947ade5778cd6c8
(cherry picked from commit
660620dd45dc11f2d889add79bedf2dc771c7d04)
Reviewed-on: https://go-review.googlesource.com/c/go/+/526575
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
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