[release-branch.go1.15] cmd/link: fix handling of dupok mapzero syms

The compiler machinery that generates "map.zero" symbols marks them as
RODATA and DUPOK, which is problematic when a given application has
multiple map zero symbols (from different packages) with varying
sizes: the dupok path in the loader assumes that if two symbols have
the same name, it is safe to pick any of the versions. In the case of
map.zero, the link needs to select the largest symbol, not an
arbitrary sym.

This patch changes the linker's dupok symbol loading path to detect
this problem, and in situations where we're loading a dupok symbol
whose name is the same as an existing symbol but whose size is large,
select the new dup over the old. Note: this fix differs from the one
used in 1.16/1.17, which uses content-addressable symbols instead.

Fixes #46656.

Change-Id: Iabd2feef01d448670ba795c7eaddc48c191ea276
Reviewed-on: https://go-review.googlesource.com/c/go/+/326211
Trust: Than McIntosh <thanm@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
(cherry picked from commit aa5540cd82170f82c6fe11511e12de96aa58cbc1)
Reviewed-on: https://go-review.googlesource.com/c/go/+/326213

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