This adds more styling to the disassemble command. In particular,
addresses and function names in the disassembly are now styled.
This required fixing a small latent bug in set_output_style. This
function always passed NULL to emit_style_escape; but when writing to
a file other than gdb_stdout, it should emit the style escape
directly. (FWIW this is another argument for better integrating the
pager with ui_file and getting rid of this entire layer.)
gdb/ChangeLog
2019-08-06 Tom Tromey <tom@tromey.com>