GDB, gdbserver: Convert regcache_register_size function to method

GDB, gdbserver: Convert regcache_register_size function to method

The regcache_register_size function has one implementation in GDB, and
one in gdbserver.  Both of them have a gdb::checked_static_cast to their
corresponding regcache class.  This can be avoided by defining a
pure virtual register_size method in the
reg_buffer_common class, which is then implemented by the reg_buffer
class in GDB, and by the regcache class in gdbserver.

Calls to the register_size () function from methods of classes in the
reg_buffer_common hierarchy need to be changed to calls to the newly
defined method, otherwise the compiler complains that a matching method
cannot be found.

Co-Authored-By: Simon Marchi <simon.marchi@efficios.com>
Approved-By: Simon Marchi <simon.marchi@efficios.com>
Reviewed-By: Tankut Baris Aktemur <tankut.baris.aktemur@intel.com>
Change-Id: I7f4f74a51e96c42604374e87321ca0e569bc07a3