LoongArch: Add support for D frontend.

gcc/ChangeLog:

	* config.gcc: Add loongarch-d.o to d_target_objs for LoongArch
	architecture.
	* config/loongarch/t-loongarch: Add object target for loongarch-d.cc.
	* config/loongarch/loongarch-d.cc
	(loongarch_d_target_versions): add interface function to define builtin
	D versions for LoongArch architecture.
	(loongarch_d_handle_target_float_abi): add interface function to define
	builtin D traits for LoongArch architecture.
	(loongarch_d_register_target_info): add interface function to register
	loongarch_d_handle_target_float_abi function.
	* config/loongarch/loongarch-d.h
	(loongarch_d_target_versions): add function prototype.
	(loongarch_d_register_target_info): Likewise.

libphobos/ChangeLog:

	* configure.tgt: Enable libphobos for LoongArch architecture.
	* libdruntime/gcc/sections/elf.d: Add TLS_DTV_OFFSET constant for
	LoongArch64.
	* libdruntime/gcc/unwind/generic.d: Add __aligned__ constant for
	LoongArch64.