[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/threadalloc.h

@@ -18,7 +18,7 @@ namespace snmalloc
    * replace itself with the thread-local allocator, allocating one if
    * required.  This avoids a branch on the fast path.
    */
-  HEADER_GLOBAL Alloc GlobalPlaceHolder(
+  inline Alloc GlobalPlaceHolder(
     default_memory_provider, SNMALLOC_DEFAULT_PAGEMAP(), nullptr, true);
 
 #ifdef SNMALLOC_EXTERNAL_THREAD_ALLOC