[gn build] Make libcxx_enable_debug_mode work better, maybe

Refer to _LIBCPP_ENABLE_DEBUG_MODE instead of the old _LIBCPP_DEBUG
in a comment, and write that to __config_site correctly too.

See 13ea1343 and the comments in https://crbug.com/1358646.

Also change the default of libcxx_enable_debug_mode to false for now.
Since we used to not write _LIBCPP_ENABLE_DEBUG_MODE, the previous
default of true had no effect (except for compiling debug.cpp and
legacy_debug_handler.cpp, which we now no longer build by default).
So this (mostly) preserves previous behavior.