gdb: move regcache::regcaches to regcache.c

I don't really understand why `regcache_thread_ptid_changed` is a static
method of `struct regcache` instead of being a static free function in
regcache.c.  And I don't understand why `current_regcache` is a static
member of `struct regcache` instead of being a static global in
regcache.c.  It's not wrong per-se, but there's no other place where we
do it like this in GDB (as far as I remember) and it just exposes things
unnecessarily in the .h.

Move them to be just static in regcache.c.  As a result,
registers_changed_ptid doesn't need to be friend of the regcache class
anymore.

Removing the include of forward_list in regcache.h showed that we were
missing an include for it in dwarf2/index-write.c, record-btrace.c and
sparc64-tdep.c.

gdb/ChangeLog:

	* regcache.h (class regcache): Remove friend
	registers_changed_ptid.
	<regcache_thread_ptid_changed>: Remove.
	<regcaches>: Remove.
	* regcache.c (regcache::regcaches): Rename to...
	(regcaches): ... this.  Make static.
	(get_thread_arch_aspace_regcache): Update.
	(regcache::regcache_thread_ptid_changed): Rename to...
	(regcache_thread_ptid_changed): ... this.  Update.
	(class regcache_access): Remove.
	(regcaches_test): Update.
	(_initialize_regcache): Update.
	* sparc64-tdep.c, dwarf2/index-write.c, record-btrace.c: Include
	<forward_list>.

Change-Id: Iabc25759848010cfbb7ee7e27f60eaca17d61c12

gdb/dwarf2/index-write.c

@@ -41,6 +41,7 @@
 
 #include <algorithm>
 #include <cmath>
+#include <forward_list>
 #include <set>
 #include <unordered_map>
 #include <unordered_set>