GDB: trad-frame: Store length of value_bytes in trad_frame_saved_reg

The goal is to ensure that it is available in frame_unwind_got_bytes () to
make sure that the provided buf isn't larger than the size of the register
being provisioned.

In the process, regcache's cached_reg_t::data also needed to be
converted to a gdb::byte_vector, so that the register contents' size can
be tracked.

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