Removes the implementation of __traits(argTypes), which only supported
x86_64 targets.  The only use of this trait is an unused va_arg()
function, this has been removed as well.

Reviewed-on: https://github.com/dlang/dmd/pull/11022

gcc/d/ChangeLog:

2020-04-13  Iain Buclaw  <ibuclaw@gdcproject.org>

	* Make-lang.in (D_FRONTEND_OBJS): Remove d/argtypes.o.
	* d-target.cc (Target::toArgTypes): New function.

libphobos/ChangeLog:

2020-04-13  Iain Buclaw  <ibuclaw@gdcproject.org>

	* libdruntime/core/stdc/stdarg.d: Remove run-time va_list template.