libstdc++: Disable floating-point std::to_chars on unsupported targets

libstdc++: Disable floating-point std::to_chars on unsupported targets

This patch conditionally disables the floating-point std::to_chars
implementation on targets whose float and double aren't IEEE binary32
and binary64, until a proper fallback can be added for such targets.
This fixes a bootstrap failure on non-IEEE-754 FP targets such as
vax-netbsdelf.

The new preprocessor tests in c++config that detect the binary32 and
binary64 formats were copied from gcc/testsuite/gcc.dg/float-exact-1.c.

libstdc++-v3/ChangeLog:

	* include/bits/c++config (_GLIBCXX_FLOAT_IS_IEEE_BINARY_32):
	Define this macro.
	(_GLIBCXX_DOUBLE_IS_IEEE_BINARY_64): Likewise.
	* include/std/charconv (to_chars): Use these macros to
	conditionally hide the overloads for floating-point types.
	* src/c++17/floating_to_chars.cc: Use the macros to
	conditionally disable this file.
	(floating_type_traits<float>): Remove redundant static assert.
	(floating_type_traits<double>): Likewise.
	* testsuite/20_util/to_chars/double.cc: Run this test only on
	ieee-floats effective targets.
	* testsuite/20_util/to_chars/float.cc: Likewise.
	* testsuite/20_util/to_chars/long_double.cc: Likewise.
	* testsuite/lib/libstdc++.exp
	(check_effective_target_ieee-floats): Define new proc for
	detecting whether float and double have the IEEE binary32 and
	binary64 formats.