[NFC] Replace HEADER_GLOBAL with inline (C++17).

HEADER_GLOBAL was using non-standard attributes to achieve what C++17
now permits with a keyword.  Use the standard formulation.

Update the README to note that gcc is still not recommended, but because
of its poor codegen for 128-bit atomic compare and exchange, rather than
because it doesn't support the attribute used for HEADER_GLOBAL.

src/mem/largealloc.h

@@ -405,5 +405,5 @@ namespace snmalloc
    * The memory provider that will be used if no other provider is explicitly
    * passed as an argument.
    */
-  HEADER_GLOBAL GlobalVirtual default_memory_provider;
+  inline GlobalVirtual default_memory_provider;
 } // namespace snmalloc