gdbserver: fix the declared type of register_status in regcache

gdbserver: fix the declared type of register_status in regcache

The register_status field of regcache is declared as `unsigned char *`.
This is incorrect, because `enum register_status` from
gdbsupport/common-regcache.h is based on signed char and
REG_UNAVAILABLE is defined as -1.  Fix the declared type.

Now that we are modifying the declaration, also use a unique_ptr
and make the field private.

The get/set methods already use the correct type, but we update cast
operations in two places.

Approved-By: Simon Marchi <simon.marchi@efficios.com>