Add support for both ISO/IEC `_Float128` and GCC `__float128` with fallback detection

[wszqkzqk]

The current compiler implementation of fp128 is confusing. __float128 is a GCC extension with inconsistent support (e.g., LoongArch64 doesn't support Q suffix literals), while _Float128 is standardized but may lack support in some compilers (e.g., Clang lacks C23 _Float128 and f128 suffixes; see llvm/llvm-project#80195).

We might implement dual support for compatibility: prioritize _Float128 detection first, then fall back to __float128 if unavailable.

See also:

• https://en.cppreference.com/w/cpp/types/floating-point
• https://gcc.gnu.org/onlinedocs/gcc/Floating-Types.html
• Related issue on gcc: GCC Bugzilla: LoongArch: Q Suffix for __float128 Literals Not Supported
• Related issue on clang: [clang] missing support for _Float128 (C23) llvm/llvm-project#80195
• The support status table on cppreference:

Types Defined in header	Literal suffix	Predefined macro	C language type	bits of storage	bits of precision	bits of exponent	max exponent
float16_t	f16 or F16	STDCPP_FLOAT16_T	_Float16	16	11	5	15
float32_t	f32 or F32	STDCPP_FLOAT32_T	_Float32	32	24	8	127
float64_t	f64 or F64	STDCPP_FLOAT64_T	_Float64	64	53	11	1023
float128_t	f128 or F128	STDCPP_FLOAT128_T	_Float128	128	113	15	16383
bfloat16_t	bf16 or BF16	STDCPP_BFLOAT16_T	(N/A)	16	8	8	127