bench: Fix initialization order in registration

The initialization order of global data structures in different
implementation units is undefined. Making use of this is essentially
gambling on what the linker does, the so-called [Static initialization
order fiasco](https://isocpp.org/wiki/faq/ctors#static-init-order).

In this case it apparently worked on Linux but failed on OpenBSD and
FreeBSD.

To create it on first use, make the registration structure local to
a function.

Fixes #8910.

src/bench/bench.h

@@ -63,7 +63,8 @@ namespace benchmark {
 
     class BenchRunner
     {
-        static std::map<std::string, BenchFunction> benchmarks;
+        typedef std::map<std::string, BenchFunction> BenchmarkMap;
+        static BenchmarkMap &benchmarks();
 
     public:
         BenchRunner(std::string name, BenchFunction func);