d: Add TARGET_D_HAS_STDCALL_CONVENTION

d: Add TARGET_D_HAS_STDCALL_CONVENTION

This replaces the use of the D front-end `is64bit' parameter in
determining whether to insert the "stdcall" function attribute.

It is also used to determine whether `extern(System)' should be the same
as `extern(Windows)' in the implementation of Target::systemLinkage.

gcc/ChangeLog:

	* config/i386/i386-d.c (ix86_d_has_stdcall_convention): New function.
	* config/i386/i386-protos.h (ix86_d_has_stdcall_convention): Declare.
	* config/i386/i386.h (TARGET_D_HAS_STDCALL_CONVENTION): Define.
	* doc/tm.texi: Regenerate.
	* doc/tm.texi.in (D language and ABI): Add @hook for
	TARGET_D_HAS_STDCALL_CONVENTION.

gcc/d/ChangeLog:

	* d-target.cc (Target::systemLinkage): Return LINKwindows if
	d_has_stdcall_convention applies to LINKsystem.
	* d-target.def (d_has_stdcall_convention): New hook.
	* types.cc (TypeVisitor::visit (TypeFunction *)): Insert "stdcall"
	function attribute if d_has_stdcall_convention applies to LINKwindows.