Change type of reg_buffer::m_register_status to register_status

The type of reg_buffer::m_register_status is an array of signed char,
probably to ensure that each element takes up only one byte.  Instead,
since we use C++11, we can force the underlying type of register_status
to be signed char and use the enum type.

gdb/ChangeLog:

	* common/common-regcache.h (enum register_status): Add
	underlying type "signed char".
	* regcache.h (reg_buffer) <m_register_status>: Change type to
	register_status *.
	* regcache.c (reg_buffer::reg_buffer): Alocate arrays of
	register_status instead of signed char.
	(reg_buffer::save): Use REG_UNKNOWN instead of 0.
	(reg_buffer::get_register_status): Remove cast.
	(readable_regcache::raw_read): Remove cast.
	(readable_regcache::cooked_read): Remove cast.

gdb/regcache.h

@@ -177,7 +177,7 @@ protected:
   /* The register buffers.  */
   gdb_byte *m_registers;
   /* Register cache status.  */
-  signed char *m_register_status;
+  register_status *m_register_status;
 
   friend class regcache;
   friend class detached_regcache;