[dev.link] cmd/link: free loader memory after LoadFull
After LoadFull, we'll be using sym.Symbols mostly. We still need
the loader information for symbol index mappings and name
lookups, but not much else. Free some memory.
Linking cmd/compile,
name old time/op new time/op delta
Loadlibfull_GC 44.5M ± 0% 35.8M ± 0% -19.66% (p=0.008 n=5+5)
Archive_GC 46.4M ± 0% 37.6M ± 0% -18.89% (p=0.008 n=5+5)
Linking cmd/compile with external linking,
name old time/op new time/op delta
Loadlibfull_GC 82.5M ± 0% 57.4M ± 0% -30.41% (p=0.008 n=5+5)
Archive_GC 86.8M ± 0% 61.7M ± 0% -28.90% (p=0.008 n=5+5)