wchar_t does not satisfy std::integral concept on DJGPP MSDOS toolchains

[Jonathan Wakely]

On 01/11/20 10:03 +0000, Jonathan Wakely wrote:
>On 31/10/20 22:21 +0000, sotrdg sotrdg via Libstdc++ wrote:
>>Please fix it.
>
>It's determined by the result of the GLIBCXX_ENABLE_WCHAR_T checks in
>libstdc++-v3/acinclude.m4
>
>djgpp must be missing some wchar_t library support.

This patch makes some of the type traits work without libc support for
wchar_t. This is necessary for freestanding anyway, because we almost
certainly don't have <wchar.h> etc. in freestanding, but the standard
still requires wchar_t to exist and be an integral type.

Tested powerpc64le-linux and (briefly) avr. For avr the value of
is_integral<wchar_t> is now true, and make_signed/unsigned work for
wchar_t.

Pushed to trunk.

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