GDB: Use gdb::array_view for buffers used in register reading and unwinding

GDB: Use gdb::array_view for buffers used in register reading and unwinding

This allows checking the size of the given buffer.  Changes
frame_register_unwind (), frame_unwind_register (), get_frame_register ()
and deprecated_frame_register_read ().

As pointed out by Baris, in the case of MIPS target code this is best
done by changing a couple of alloca-based buffers in
mips_read_fp_register_single and mips_print_fp_register to
gdb::byte_vector instances.

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