[GOLD] PR27815, gold fails to build with latest GCC

Don't use nullptr, it requires -std=c++11 on versions of gcc prior to
6.1.  It would be possible to arrange to pass -std=c++11 automatically
when required (top level configure does that for gcc builds) but that
seems overkill and since we're not up-to-date on the top level config
files would mean someone would need to sync those over.

	PR gold/27815
	* gc.h (gc_process_relocs): Use cast in Section_id constructor.