libstdc++: Add std::experimental::simd from the Parallelism TS 2

libstdc++: Add std::experimental::simd from the Parallelism TS 2

Adds <experimental/simd>.

This implements the simd and simd_mask class templates via
[[gnu::vector_size(N)]] data members. It implements overloads for all of
<cmath> for simd. Explicit vectorization of the <cmath> functions is not
finished.

The majority of functions are marked as [[gnu::always_inline]] to enable
quasi-ODR-conforming linking of TUs with different -m flags.
Performance optimization was done for x86_64.  ARM, Aarch64, and POWER
rely on the compiler to recognize reduction, conversion, and shuffle
patterns.

Besides verification using many different machine flages, the code was
also verified with different fast-math flags.

libstdc++-v3/ChangeLog:

	* doc/xml/manual/status_cxx2017.xml: Add implementation status
	of the Parallelism TS 2. Document implementation-defined types
	and behavior.
	* include/Makefile.am: Add new headers.
	* include/Makefile.in: Regenerate.
	* include/experimental/simd: New file. New header for
	Parallelism TS 2.
	* include/experimental/bits/numeric_traits.h: New file.
	Implementation of P1841R1 using internal naming. Addition of
	missing IEC559 functionality query.
	* include/experimental/bits/simd.h: New file. Definition of the
	public simd interfaces and general implementation helpers.
	* include/experimental/bits/simd_builtin.h: New file.
	Implementation of the _VecBuiltin simd_abi.
	* include/experimental/bits/simd_converter.h: New file. Generic
	simd conversions.
	* include/experimental/bits/simd_detail.h: New file. Internal
	macros for the simd implementation.
	* include/experimental/bits/simd_fixed_size.h: New file. Simd
	fixed_size ABI specific implementations.
	* include/experimental/bits/simd_math.h: New file. Math
	overloads for simd.
	* include/experimental/bits/simd_neon.h: New file. Simd NEON
	specific implementations.
	* include/experimental/bits/simd_ppc.h: New file. Implement bit
	shifts to avoid invalid results for integral types smaller than
	int.
	* include/experimental/bits/simd_scalar.h: New file. Simd scalar
	ABI specific implementations.
	* include/experimental/bits/simd_x86.h: New file. Simd x86
	specific implementations.
	* include/experimental/bits/simd_x86_conversions.h: New file.
	x86 specific conversion optimizations. The conversion patterns
	work around missing conversion patterns in the compiler and
	should be removed as soon as PR85048 is resolved.
	* testsuite/experimental/simd/standard_abi_usable.cc: New file.
	Test that all (not all fixed_size<N>, though) standard simd and
	simd_mask types are usable.
	* testsuite/experimental/simd/standard_abi_usable_2.cc: New
	file. As above but with -ffast-math.
	* testsuite/libstdc++-dg/conformance.exp: Don't build simd tests
	from the standard test loop. Instead use
	check_vect_support_and_set_flags to build simd tests with the
	relevant machine flags.