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

...gold/gc.h:250:37: error: ISO C++ says that these are ambiguous, even though the worst conversion for the first is better than the worst conversion for the second: [-Werror]
  250 |                 (*secvec).push_back(Section_id(NULL, 0));
      |                                     ^~~~~~~~~~~~~~~~~~~

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

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.

https://en.cppreference.com/w/cpp/types/NULL says NULL might be
defined as nullptr.
https://en.cppreference.com/w/cpp/language/reinterpret_cast says
reinterpret_cast can't be used on nullptr.

	PR gold/28106
	PR gold/27815
	* gc.h (gc_process_relocs): Use static_cast in Section_id constructor.

(cherry picked from commit 1f1fb219)
(cherry picked from commit 5d7f11f0)
(cherry picked from commit b97bd976)