Use gdbarch obstack to allocate the TYPE_NAME string in arch_type

Use gdbarch obstack to allocate the TYPE_NAME string in arch_type

Since the type whose name is being set is now being allocated on the
gdbarch obstack, we should allocate its TYPE_NAME on the obstack too.
This reduces the number of individual valgrind warnings for the command
"gdb gdb" from ~300 to ~150.

Tested on x86_64-unknown-linux-gnu.

gdb/ChangeLog:

	* gdbarch.h (gdbarch_obstack_strdup): Declare.
	* gdbarch.c (gdbarch_obstack_strdup): Define.
	* gdbtypes.c (arch_type): Use it.