libstdc++: Disable "ALT128" long double support for Clang

libstdc++: Disable "ALT128" long double support for Clang

Clang does not currently support the __ibm128 type [1] and only supports
the __ieee128 type in the unreleased 12.0.0 version [2]. That means it
is not possible to provide support for -mabi=ieeelongdouble with Clang
in an ABI compatible way (as we do for GCC by defining new facets and
other types in the __gnu_cxx_ldbl128 namespace).

By preventing the definition of _GLIBCXX_LONG_DOUBLE_ALT128_COMPAT when
compiling with Clang, all uses of __ibm128 and __ieee128 types will be
disabled. This can be revisited in future when Clang supports the types
(and provides a way to detect that support using the preprocessor).

[1] https://reviews.llvm.org/D93377
[2] https://reviews.llvm.org/D97846

libstdc++-v3/ChangeLog:

	* include/bits/c++config (_GLIBCXX_LONG_DOUBLE_ALT128_COMPAT):
	Do not define when compiling with Clang.