keymap-gen: add support for generating output for C++

Designated initializers ([index] = value,) are not available in C++:

https://gcc.gnu.org/onlinedocs/gcc/Designated-Inits.html

So add a custom stdc++ output format which outputs all array entries instead of the "sparse" C variant.

Signed-off-by: Peter Korsgaard peter@korsgaard.com