gdbserver: no point in hiding the regcache type nowadays

gdbserver: no point in hiding the regcache type nowadays

The regcache used to be hidden inside inferiors.c, but since the
tracepoints support that it's a first class object.  This also fixes a
few implicit pointer conversion errors in C++ mode, caused by a few
places missing the explicit cast.

gdb/gdbserver/ChangeLog:
2015-08-06  Pedro Alves  <palves@redhat.com>

	* gdbthread.h (struct regcache): Forward declare.
	(struct thread_info) <regcache_data>: Now a struct regcache
	pointer.
	* inferiors.c (inferior_regcache_data)
	(set_inferior_regcache_data): Now work with struct regcache
	pointers.
	* inferiors.h (struct regcache): Forward declare.
	(inferior_regcache_data, set_inferior_regcache_data): Now work
	with struct regcache pointers.
	* regcache.c (get_thread_regcache, regcache_invalidate_thread)
	(free_register_cache_thread): Remove struct regcache pointer
	casts.