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,
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.