aarch64: Add DLL import/export to AArch64 target

aarch64: Add DLL import/export to AArch64 target

This patch reuses the MinGW implementation to enable DLL import/export
functionality for the aarch64-w64-mingw32 target. It also modifies
environment configurations for MinGW.

2024-06-08  Evgeny Karpov <Evgeny.Karpov@microsoft.com>

	gcc/ChangeLog:
	* config.gcc: Add winnt-dll.o, which contains the DLL
	import/export implementation.
	* config/aarch64/aarch64.cc (aarch64_load_symref_appropriately):
	Add dllimport implementation.
	(aarch64_expand_call): Likewise.
	(aarch64_legitimize_address): Likewise.
	* config/aarch64/cygming.h (SYMBOL_FLAG_DLLIMPORT): Modify MinGW
	environment to support DLL import/export.
	(SYMBOL_FLAG_DLLEXPORT): Likewise.
	(SYMBOL_REF_DLLIMPORT_P): Likewise.
	(SYMBOL_FLAG_STUBVAR): Likewise.
	(SYMBOL_REF_STUBVAR_P): Likewise.
	(TARGET_VALID_DLLIMPORT_ATTRIBUTE_P): Likewise.
	(TARGET_ASM_FILE_END): Likewise.
	(SUB_TARGET_RECORD_STUB): Likewise.
	(GOT_ALIAS_SET): Likewise.
	(PE_COFF_EXTERN_DECL_SHOULD_BE_LEGITIMIZED): Likewise.
	(HAVE_64BIT_POINTERS): Likewise.