Fix compilation errors with libstdc++v3 for AVR target and allow --enable-libstdcxx

[Jonathan Wakely]

On 10/11/16 13:51 -0200, Felipe Magno de Almeida wrote:
>Subject: [PATCH 1/3] Add #ifdef case for 16 bits in cow-stdexcept.cc
>
>Added #ifdef case for when void* is 16 bits so it compiles in AVR
>target.

I've committed this patch.

>From 7ed4af72fe0bdee1a38c7487955590fb64f76a5d Mon Sep 17 00:00:00 2001
>From: Felipe Magno de Almeida <felipe@expertisesolutions.com.br>
>Date: Thu, 15 Sep 2016 18:52:57 -0300
>Subject: [PATCH 2/3] Use temporary int objects to access struct tm members
>
>Call _M_extract_* functions family through temporary int objects, so
>it doesn't convert from lvalue to rvalue through a temporary in AVR
>because of the incompatible types used in AVR-Libc.
>
>This fixes compilation errors with AVR-Libc while compiling libstdc++
>for AVR target.

This one has problems, see my earlier email.


>Subject: [PATCH 3/3] Enable libstdc++ compilation in AVR targets
>
>Enable libstdc++ compilation in AVR targets with AVR-Libc. Most
>floating point math functions are already defined in AVR-Libc, so
>defines are in place to avoid multiple definition of these functions.

I've committed this one too.