gdb/python: new gdb.architecture_names function

Add a new function to the Python API, gdb.architecture_names().  This
function returns a list containing all of the supported architecture
names within the current build of GDB.

The values returned in this list are all of the possible values that
can be returned from gdb.Architecture.name().

gdb/python/python.c

@@ -2324,6 +2324,10 @@ Set the value of the convenience variable $NAME." },
 Register a TUI window constructor." },
 #endif	/* TUI */
 
+  { "architecture_names", gdbpy_all_architecture_names, METH_NOARGS,
+    "architecture_names () -> List.\n\
+Return a list of all the architecture names GDB understands." },
+
   {NULL, NULL, 0, NULL}
 };