systemd: sync with upstream live ebuild, enable gold linker

Minor sync with upstream, adds a use flag we don't enable.

Updated our toolchain just in time, the -fuse-ld= option is now
supported and as of 218 systemd doesn't link with bfd any more so now is
a good time to re-enable the configure test to enable it. For reference
the guilty commit at fault is probably:

https://github.com/systemd/systemd/commit/5f86c1f4c43ee9caa120d130e9b89d3fd25124c0

Or specifically:

```c
/* GCC maps this magically to the beginning and end of the BUS_ERROR_MAP section */
extern const sd_bus_error_map __start_BUS_ERROR_MAP[];
extern const sd_bus_error_map __stop_BUS_ERROR_MAP[];
```

... to which I say haha wtf... WELL WHAT ABOUT WHEN IT DOESN'T? :-P
systemd: bump to 218
Merge pull request #1019 from marineam/systemd

Systemd 218