toolchain/53679: clang++ AND netbsd AND libstdc++ AND c++11 is dysfunctional due to __float128

[coypu%sdf.org@localhost]

>Number:         53679
>Category:       toolchain
>Synopsis:       clang++ AND netbsd AND libstdc++ AND c++11 is dysfunctional due to __float128
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    toolchain-manager
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Mon Oct 22 14:10:00 +0000 2018
>Originator:     coypu
>Release:        NetBSD 8.99.21
>Organization:
>Environment:
NetBSD planets 8.99.21 NetBSD 8.99.21 (GENERIC) #0: Fri Jul 13 14:49:46 IDT 2018  fly@planets:/tmp/build3/sys/arch/amd64/compile/GENERIC amd64
>Description:
> cat test3.cxx
#include <algorithm>
int main() { return 0; }

> clang++ --version # this is from pkgsrc
clang version 6.0.1 (tags/RELEASE_601/final)
Target: x86_64-unknown-netbsd8.99
Thread model: posix
InstalledDir: /usr/pkg/bin

> clang++ test3.cxx 
In file included from test3.cxx:1:
In file included from /usr/include/g++/algorithm:60:
In file included from /usr/include/g++/utility:70:
In file included from /usr/include/g++/bits/stl_pair.h:59:
In file included from /usr/include/g++/bits/move.h:57:
/usr/include/g++/type_traits:344:39: error: __float128 is not supported on this target
    struct __is_floating_point_helper<__float128>
                                      ^
1 error generated.


maybe in OSTargets.h in clang we need to have like in other targets
    if (this->HasFloat128)
       Builder.defineMacro("__FLOAT128__");



or maybe libstdc++ needs to check in a more standard way if one exists?
>How-To-Repeat:

>Fix: