PR28106, build of 2.37 fails on FreeBSD and Clang

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 b97bd976)