gccrs: libproc_macro: Add procmacro array definition

Standard types do not guarantee anything about their ABI and are
therefore unconvenient to use as a primarily exposed container.
With such asumption this commit introduces a container.

libgrust/ChangeLog:

	* libproc_macro/proc_macro.h (struct ProcmacroArray):
	Add ProcmacroArray definition.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>

libgrust/libproc_macro/proc_macro.h

@@ -89,6 +89,12 @@ public:
   Procmacro make_bang (const char *name, BangMacro macro);
 };
 
+struct ProcmacroArray
+{
+  std::uint64_t length;
+  Procmacro *macros;
+};
+
 } // namespace ProcMacro
 
 #endif /* ! PROC_MACRO_H */