gas: default .debug section compression method adjustments

gas: default .debug section compression method adjustments

While commit b0c295e1b8d0 ("add --enable-default-compressed-debug-
sections-algorithm configure option") adjusted flag_compress_debug's
initializer, it didn't alter the default used when the command line
option was specified with an (optional!) argument. This rendered help
text inconsistent with actual behavior in certain configurations.

As to help text - the default reported there clearly shouldn't be
affected by a possible earlier --compress-debug-sections= option, so
flag_compress_debug can't be used when emitting usage information.