aarch64: Add bfloat16_t support for aarch64

x86_64/i686 has for a few months working std::bfloat16_t support, __bf16
there is no longer a storage only type, but can be used for arithmetics
and is supported in libgcc and libstdc++.

The following patch adds similar support for AArch64.

Unlike the x86 changes, this one keeps the old __bf16 mangling of
u6__bf16 rather than DF16b (so an exception from Itanium ABI), but
otherwise __bf16 and decltype (0.0bf16) are the same type and both
in C++ act as extended floating-point type.

2023-03-13  Jakub Jelinek  <jakub@redhat.com>

gcc/
	* config/aarch64/aarch64.h (aarch64_bf16_type_node): Remove.
	(aarch64_bf16_ptr_type_node): Adjust comment.
	* config/aarch64/aarch64.cc (aarch64_gimplify_va_arg_expr): Use
	bfloat16_type_node rather than aarch64_bf16_type_node.
	(aarch64_libgcc_floating_mode_supported_p,
	aarch64_scalar_mode_supported_p): Also support BFmode.
	(aarch64_invalid_conversion, aarch64_invalid_unary_op): Remove.
	(aarch64_invalid_binary_op): Remove BFmode related rejections.
	(TARGET_INVALID_CONVERSION, TARGET_INVALID_UNARY_OP): Don't redefine.
	* config/aarch64/aarch64-builtins.cc (aarch64_bf16_type_node): Remove.
	(aarch64_int_or_fp_type): Use bfloat16_type_node rather than
	aarch64_bf16_type_node.
	(aarch64_init_simd_builtin_types): Likewise.
	(aarch64_init_bf16_types): Likewise.  Don't create bfloat16_type_node,
	which is created in tree.cc already.
	* config/aarch64/aarch64-sve-builtins.def (svbfloat16_t): Likewise.
gcc/testsuite/
	* gcc.target/aarch64/sve/acle/general-c/ternary_bfloat16_opt_n_1.c:
	Don't expect one __bf16 related error.
	* gcc.target/aarch64/bfloat16_vector_typecheck_1.c: Adjust or remove
	dg-error directives for __bf16 being an extended arithmetic type.
	* gcc.target/aarch64/bfloat16_vector_typecheck_2.c: Likewise.
	* gcc.target/aarch64/bfloat16_scalar_typecheck.c: Likewise.
	* g++.target/aarch64/bfloat_cpp_typecheck.C: Don't expect two __bf16
	related errors.
libgcc/
	* config/aarch64/t-softfp (softfp_extensions): Add bfsf.
	(softfp_truncations): Add tfbf dfbf sfbf hfbf.
	(softfp_extras): Add floatdibf floatundibf floattibf floatuntibf.
	* config/aarch64/libgcc-softfp.ver (GCC_13.0.0): Export
	__extendbfsf2 and __trunc{s,d,t,h}fbf2.
	* config/aarch64/sfp-machine.h (_FP_NANFRAC_B, _FP_NANSIGN_B): Define.
	* soft-fp/floatundibf.c: New file.
	* soft-fp/floatdibf.c: New file.
libstdc++-v3/
	* config/abi/pre/gnu.ver (CXXABI_1.3.14): Also export __bf16 tinfos
	if it isn't mangled as DF16b but u6__bf16.