[Solaris] __float128 is supported on Solaris/x86

When rebasing https://reviews.llvm.org/D40898 with GCC 5.4 on Solaris 11.4, I ran
into a few instances of

In file included from /vol/llvm/src/compiler-rt/local/test/asan/TestCases/Posix/asan-symbolize-sanity-test.cc:19:
In file included from /usr/gcc/5/lib/gcc/x86_64-pc-solaris2.11/5.4.0/../../../../include/c++/5.4.0/string:40:
In file included from /usr/gcc/5/lib/gcc/x86_64-pc-solaris2.11/5.4.0/../../../../include/c++/5.4.0/bits/char_traits.h:39:
In file included from /usr/gcc/5/lib/gcc/x86_64-pc-solaris2.11/5.4.0/../../../../include/c++/5.4.0/bits/stl_algobase.h:64:
In file included from /usr/gcc/5/lib/gcc/x86_64-pc-solaris2.11/5.4.0/../../../../include/c++/5.4.0/bits/stl_pair.h:59:
In file included from /usr/gcc/5/lib/gcc/x86_64-pc-solaris2.11/5.4.0/../../../../include/c++/5.4.0/bits/move.h:57:
/usr/gcc/5/lib/gcc/x86_64-pc-solaris2.11/5.4.0/../../../../include/c++/5.4.0/type_traits:311:39: error: __float128 is not supported on this target

  struct __is_floating_point_helper<__float128>
                                    ^

during make check-all.  The line above is inside

#if !defined(__STRICT_ANSI__) && defined(_GLIBCXX_USE_FLOAT128)

  template<>
    struct __is_floating_point_helper<__float128>
    : public true_type { };

#endif

While the libstdc++ header indicates support for __float128, clang does not, but
should.  The following patch implements this and fixed those errors.

Differential Revision: https://reviews.llvm.org/D41240

llvm-svn: 330572