Fix compilation with avr-gcc 7x

[adbancroft]

Re-instate https://github.com/modm-io/avr-libstdcpp/commit/1e245c13f7f09f2469f123e5a58c1e8d082a6825, which was removed as part of https://github.com/modm-io/avr-libstdcpp/commit/4fd647b07f4a87242ee378df303401fbfbe45e00.

This provides support for avr-gcc 7.x: this is the gcc version that Arduino ships with, so it's a popular platform.

[adbancroft]

May help #15, though avr-gcc 5.2 may be too out of date to parse some of the library.

[ckormanyos]

I just wanted to bump this item. Thanks @adbancroft this does interest us. I'll try to scrape up just a bit of time to take it down the GCC5 road and see what's up. I intend to look at it. It's just that this project is hard for me to find time for.

Cc: @salkinium

[salkinium]

So sorry for the wait, I completely forgot about this. I'll merge this on the weekend if there are no reviews by then.

[ckormanyos]

sorry for the wait, I completely forgot about this.

As did I also forget. Sorry also. Thank you @salkinium and @adbancroft for reporting this and working it out.