target_stack -> current_top_target() throughout

The recent C++ification of target_ops replaced references to the old
"current_target" squashed target throughout with references to a
"target_stack" pointer.  I had picked the "target_stack" name very
early in the multi-target work, and managed to stick with it, even
though it's a bit of a misnomer, since it isn't really a "target
stack" object, but a pointer into the current top target in the stack.
As I'm splitting more pieces off of the multi-target branch, I've come
to think that it's better to rename it now.  A following patch will
introduce a new class to represent a target stack, and "target_stack"
would be _its_ ideal name.  (In the branch, the class is called
a_target_stack to work around the clash.)

Thus this commit renames target_stack to current_top_target and
replaces all references throughout.  Also, while at it,
current_top_target is made a function instead of a pointer, to make it
possible to change its internal implementation without leaking
implementation details out.  In a couple patches, the implementation
of the function will change to refer to a target stack object, and
then further down the multi-target work, it'll change again to find
the right target stack for the current inferior.

gdb/ChangeLog:
2018-06-07  Pedro Alves  <palves@redhat.com>

	* target.h (target_stack): Delete.
	(current_top_target): Declare function.
	* target.c (target_stack): Delete.
	(g_current_top_target): New.
	(current_top_target): New function.
	* auxv.c: Use current_top_target instead of target_stack
	throughout.
	* avr-tdep.c: Likewise.
	* breakpoint.c: Likewise.
	* corefile.c: Likewise.
	* elfread.c: Likewise.
	* eval.c: Likewise.
	* exceptions.c: Likewise.
	* frame.c: Likewise.
	* gdbarch-selftests.c: Likewise.
	* gnu-v3-abi.c: Likewise.
	* ia64-tdep.c: Likewise.
	* ia64-vms-tdep.c: Likewise.
	* infcall.c: Likewise.
	* infcmd.c: Likewise.
	* infrun.c: Likewise.
	* linespec.c: Likewise.
	* linux-tdep.c: Likewise.
	* minsyms.c: Likewise.
	* ppc-linux-nat.c: Likewise.
	* ppc-linux-tdep.c: Likewise.
	* procfs.c: Likewise.
	* regcache.c: Likewise.
	* remote.c: Likewise.
	* rs6000-tdep.c: Likewise.
	* s390-linux-nat.c: Likewise.
	* s390-tdep.c: Likewise.
	* solib-aix.c: Likewise.
	* solib-darwin.c: Likewise.
	* solib-dsbt.c: Likewise.
	* solib-spu.c: Likewise.
	* solib-svr4.c: Likewise.
	* solib-target.c: Likewise.
	* sparc-tdep.c: Likewise.
	* sparc64-tdep.c: Likewise.
	* spu-tdep.c: Likewise.
	* symfile.c: Likewise.
	* symtab.c: Likewise.
	* target-descriptions.c: Likewise.
	* target-memory.c: Likewise.
	* target.c: Likewise.
	* target.h: Likewise.
	* tracefile-tfile.c: Likewise.
	* tracepoint.c: Likewise.
	* valops.c: Likewise.
	* valprint.c: Likewise.
	* value.c: Likewise.
	* windows-tdep.c: Likewise.
	* mi/mi-main.c: Likewise.

gdb/elfread.c

@@ -833,7 +833,8 @@ elf_gnu_ifunc_resolve_by_got (const char *name, CORE_ADDR *addr_p)
       if (target_read_memory (pointer_address, buf, ptr_size) != 0)
 	continue;
       addr = extract_typed_address (buf, ptr_type);
-      addr = gdbarch_convert_from_func_ptr_addr (gdbarch, addr, target_stack);
+      addr = gdbarch_convert_from_func_ptr_addr (gdbarch, addr,
+						 current_top_target ());
       addr = gdbarch_addr_bits_remove (gdbarch, addr);
 
       if (elf_gnu_ifunc_record_cache (name, addr))
@@ -900,12 +901,12 @@ elf_gnu_ifunc_resolve_addr (struct gdbarch *gdbarch, CORE_ADDR pc)
      parameter.  FUNCTION is the function entry address.  ADDRESS may be a
      function descriptor.  */
 
-  target_auxv_search (target_stack, AT_HWCAP, &hwcap);
+  target_auxv_search (current_top_target (), AT_HWCAP, &hwcap);
   hwcap_val = value_from_longest (builtin_type (gdbarch)
 				  ->builtin_unsigned_long, hwcap);
   address_val = call_function_by_hand (function, NULL, 1, &hwcap_val);
   address = value_as_address (address_val);
-  address = gdbarch_convert_from_func_ptr_addr (gdbarch, address, target_stack);
+  address = gdbarch_convert_from_func_ptr_addr (gdbarch, address, current_top_target ());
   address = gdbarch_addr_bits_remove (gdbarch, address);
 
   if (name_at_pc)
@@ -1012,7 +1013,7 @@ elf_gnu_ifunc_resolver_return_stop (struct breakpoint *b)
   resolved_address = value_as_address (value);
   resolved_pc = gdbarch_convert_from_func_ptr_addr (gdbarch,
 						    resolved_address,
-						    target_stack);
+						    current_top_target ());
   resolved_pc = gdbarch_addr_bits_remove (gdbarch, resolved_pc);
 
   gdb_assert (current_program_space == b->pspace || b->pspace == NULL);