[dev.link] cmd/link: convert mangleTypeSym to new style
Use symbol's Extname, instead of symbol renaming, for the mangled
names.
The old symbol Rename has an interesting logic of "merging"
symbols, when a symbol is renamed to the name of an existing
symbol. It turns out that this is needed for linking against
shared libraries, where the Go object has a reference to a symbol
with the original name, and the shared libary provides a symbol
under the mangled name. Implement this logic with the loader.