LoongArch: Implement 128-bit floating point functions in gcc.

LoongArch: Implement 128-bit floating point functions in gcc.

During implementation, float128_type_node is bound with the type "__float128"
so that the compiler can correctly identify the type   of the function. The
"q" suffix is associated with the "f128" function, which makes GCC more
flexible to support different user input cases, implementing functions such
as __builtin_{huge_valq, infq, fabsq, copysignq, nanq, nansq}.

gcc/ChangeLog:

	* config/loongarch/loongarch-builtins.cc (loongarch_init_builtins):
	Associate the __float128 type to float128_type_node so that it can
	be recognized by the compiler.
	* config/loongarch/loongarch-c.cc (loongarch_cpu_cpp_builtins):
	Add the flag "FLOAT128_TYPE" to gcc and associate a function
	with the suffix "q" to "f128".
	* doc/extend.texi:Added support for 128-bit floating-point functions on
	the LoongArch architecture.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/math-float-128.c: New test.