Support type _Float16/__bf16 independent of SSE2.

Support type _Float16/__bf16 independent of SSE2.

Enable _Float16 and __bf16 all the time but issue errors when the
types are used in conversion, unary operation, binary operation,
parameter passing or value return when TARGET_SSE2 is not available.

Also undef macros which are used by libgcc/libstdc++ to check the
backend support of the _Float16/__bf16 types when TARGET_SSE2 is not
available.

gcc/ChangeLog:

	PR target/109504
	* config/i386/i386-builtins.cc
	(ix86_register_float16_builtin_type): Remove TARGET_SSE2.
	(ix86_register_bf16_builtin_type): Ditto.
	* config/i386/i386-c.cc (ix86_target_macros): When TARGET_SSE2
	isn't available, undef the macros which are used to check the
	backend support of the _Float16/__bf16 types when building
	libstdc++ and libgcc.
	* config/i386/i386.cc (construct_container): Issue errors for
	HFmode/BFmode when TARGET_SSE2 is not available.
	(function_value_32): Ditto.
	(ix86_scalar_mode_supported_p): Remove TARGET_SSE2 for HFmode/BFmode.
	(ix86_libgcc_floating_mode_supported_p): Ditto.
	(ix86_emit_support_tinfos): Adjust codes.
	(ix86_invalid_conversion): Return diagnostic message string
	when there's conversion from/to BF/HFmode w/o TARGET_SSE2.
	(ix86_invalid_unary_op): New function.
	(ix86_invalid_binary_op): Ditto.
	(TARGET_INVALID_UNARY_OP): Define.
	(TARGET_INVALID_BINARY_OP): Define.
	* config/i386/immintrin.h [__SSE2__]: Remove for fp16/bf16
	related instrinsics header files.
	* config/i386/i386.h (VALID_SSE2_TYPE_MODE): New macro.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/pr109504.c: New test.
	* gcc.target/i386/sse2-bfloat16-1.c: Adjust error info.
	* gcc.target/i386/sse2-float16-1.c: Ditto.
	* gcc.target/i386/sse2-float16-4.c: New test.
	* gcc.target/i386/sse2-float16-5.c: New test.
	* g++.target/i386/float16-1.C: Adjust error info.

libgcc/ChangeLog:

	* config/i386/t-softfp: Add -msse2 to libbid HFtype related
	files.