Adjust DLL import/export implementation for AArch64

Adjust DLL import/export implementation for AArch64

The DLL import/export mingw implementation, originally from ix86,
requires minor adjustments to be compatible with AArch64.

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

	gcc/ChangeLog:
	* config/i386/cygming.h
	(PE_COFF_EXTERN_DECL_SHOULD_BE_LEGITIMIZED): Declare whether an
	external declaration should be legitimized.
	(HAVE_64BIT_POINTERS): Define whether the target supports 64-bit
	pointers.
	* config/mingw/mingw32.h (defined): Use the correct
	DllMainCRTStartup entry function.
	* config/mingw/winnt-dll.cc (defined): Exclude ix86-related code.