gdb/python: allow Architecture.disassemble to give styled output

Extend the Architecture.disassemble API to allow the user to request
styled disassembler output via a new styling argument.  A user can now
write:

  insn = arch.disassemble(address, styling = True)

The instruction strings returned within INSN will contain ANSI escape
sequences so long as 'set style enabled on' is in effect.  This means
that the user's personal settings (disabling styling) will override a
GDB extension that requests styled disassembler output.  I think this
makes sense.

The default for the styling argument is False, this maintains the
current unstyled output as default.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Approved-By: Tom Tromey <tom@tromey.com>