gdb/disasm: read opcodes bytes with a single read_code call

gdb/disasm: read opcodes bytes with a single read_code call

This commit reduces the number of times we call read_code when
printing the instruction opcode bytes during disassembly.

I've added a new gdb::byte_vector within the
gdb_pretty_print_disassembler class, in line with all the other
buffers that gdb_pretty_print_disassembler needs.  This byte_vector is
then resized as needed, and filled with a single read_code call for
each instruction.

There should be no user visible changes after this commit.