[dev.link] cmd/link: free compressed symbols after dwarfcompress

[dev.link] cmd/link: free compressed symbols after dwarfcompress

Once the symbol is compressed, we will not need the uncompressed
symbol content. Free its memory.

Linking cmd/compile,

name               old live-B     new live-B     delta
Dwarfcompress_GC      42.7M ± 0%     37.9M ± 0%   -11.31%  (p=0.008 n=5+5)

Change-Id: Ib6cc73832946d158ff4f5b4f31be9c35ba7cf103
Reviewed-on: https://go-review.googlesource.com/c/go/+/230859
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>