[dev.link] cmd/link: drop fields of external symbols after LoadFull
[dev.link] cmd/link: drop fields of external symbols after LoadFull

Free some memory.

Linking cmd/compile with external linking,

name             old live-B     new live-B     delta
Loadlibfull_GC     55.3M ± 0%     52.2M ± 0%    -5.50%  (p=0.008 n=5+5)

Change-Id: Ib8de2e61f2b012efaf6bdd98f9e5c56a19910b47
Reviewed-on: https://go-review.googlesource.com/c/go/+/231224
Reviewed-by: Jeremy Faller <b3f594e10a9edcf5413cf1190121d45078c62290@golang.org>
Reviewed-by: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>

golang/go,nawawi/go,nawawi/go,cloudflare/go,nawawi/go,golang/go,akutz/go,CAFxX/go,golang/go,nawawi/go,akutz/go,nawawi/go,golang/go,CAFxX/go,akutz/go,golang/go,golang/go,golang/go,CAFxX/go,golang/go,akutz/go,nawawi/go,cloudflare/go,CAFxX/go,cloudflare/go,nawawi/go,cloudflare/go,akutz/go,cloudflare/go,CAFxX/go,cloudflare/go,CAFxX/go,CAFxX/go,akutz/go,akutz/go,akutz/go,CAFxX/go,nawawi/go,cloudflare/go,cloudflare/go