1. Removed ABI name fiddling from the front-end.
2. Removed `Target.prefixName` and C++ tests for matching `__Z`.
3. Prepend prefixes for C++ and D symbols in the backend only
for the targets that require it (Win32, OSX).
4. Removed `RTLSYM__DINVARIANT` runtime library symbol, name is now
the same on all targets.
5. Correctly set C++ mangling on `LINK.cpp` symbols, before they
were all being set as D mangled symbols.
6. Use System mangling as synonym for do not modify symbol.
7. Added workaround for `extern(D) ___tls_get_addr`.
