libstdc++: Make use of __builtin_bit_cast for simd

libstdc++: Make use of __builtin_bit_cast for simd

The __bit_cast function was a hack to achieve what __builtin_bit_cast
can do, therefore use __builtin_bit_cast if possible. However,
__builtin_bit_cast cannot be used to cast from/to fixed_size_simd, since
it isn't trivially copyable (in the language sense — in principle it
is). Therefore add __proposed::simd_bit_cast to enable the use case
required in the test framework.

Signed-off-by: Matthias Kretz <m.kretz@gsi.de>
libstdc++-v3/ChangeLog:

	* include/experimental/bits/simd.h (__bit_cast): Implement via
	__builtin_bit_cast #if available.
	(__proposed::simd_bit_cast): Add overloads for simd and
	simd_mask, which use __builtin_bit_cast (or __bit_cast #if not
	available), which return an object of the requested type with
	the same bits as the argument.
	* include/experimental/bits/simd_math.h: Use simd_bit_cast
	instead of __bit_cast to allow casts to fixed_size_simd.
	(copysign): Remove branch that was only required if __bit_cast
	cannot be constexpr.
	* testsuite/experimental/simd/tests/bits/test_values.h: Switch
	from __bit_cast to __proposed::simd_bit_cast since the former
	will not cast fixed_size objects anymore.